The Origins of Life

Origins of Life on Earth

Although Earth was created around 4.5 billion years ago, life began to exist not long after. Due to the huge timescales involved, there is inconclusive evidence for exact dates, but nonetheless, the eagerness of life to exist was apparent from the beginning.

Our Solar System was still young, and the Sun was still cooling down after its creation billions of years beforehand. The unique circumstances of our Solar System and our planet gave rise to life. This was due to a number of characteristics that are exhibited by our ecosphere, the area of a planet capable of sustaining life.

Venus, one of our planetary neighbours, is closer to the sun, with the planet exhibiting characteristics that would not be able to support life. On the other hand, Mars is further away from the Sun, and too cold to naturally support life. However, with manipulation by man, via terraforming, Mars could indeed support life in its present state.

However, Earth, for billions of years, has possessed all the materials and suitable conditions for supporting life.


All living things possess the element carbon within them. In light of this, Earth had to have rich supply of carbon to support a rich diversity of life. This carbon was made available by the volatile nature of the Earth in the beginning, where volcanoes spewed various elements into the Earth's atmosphere.

While other elements were present, various chemical reactions began to take place which would result in the creation of new compounds and elements. One of the family of compounds created over time were the amino acids, the building blocks of protein.

Amino Acids and Proteins

Amino acids are the building blocks of protein, and thus the building blocks of life. The complex organisms of today harness the biological power of proteins in a variety of ways, such as the use of enzymes as a catalyst. (See Protein Variety and also Protein Synthesis)

The First Life

In general, organisms over time in the evolutionary chain have grown and become more complex in their nature, i.e. the first origins of life were likely small, simple and not diversified.

One understanding of the origins of life is that it would have been very unlikely that parasites were the beginnings of life. As parasites require biological hosts to reproduce and thus survive as a species, they would have been unable to successfully continue their species during this time period. In light of this, viruses and other parasites would have developed later on in the evolutionary chain.

It is believed that heterotrophs were the first beginnings of life on Earth, inhabiting the sea and absorbing the organic material that was being created by the reactions of Earth at the time (i.e. the creation of amino acids). The building blocks of life created these organisms and also acted as a food source.

This is where the idea of a food chain becomes relevant. When these first autotrophs died, the organic material that they consist of would break down and add to the 'organic soup' that was feeding these organisms at the time.

Alias, it is believed that heterotrophic bacteria was the first signs of life on Earth

Beginning of Evolution

A component of all existing life is that it adapts to survive. You either adapt or you already have adapted. If species did not have this instinctive nature via their genetic information, then they would have no desire to continue living as a species.

Although the beginnings of life above were successfully reproducing, an economy of scale involving the organisms would point out that their food source (the organic soup) would not be able to sustain all life. In light of this, the organisms on Earth at the time would have to diversify over the long term to survive.

It is suggested that around three billion years ago, autotrophic animals had diversified from previous species. These autotrophs are capable of synthesising energy from inorganic material, i.e. via the sun and elements on the Earth. This had allowed life on Earth to tap into a whole new energy resource, one that was literally inexhaustible - the Sun.

With this in hand, the snowball effect of evolution was beginning to take place....

Evolution of Life - Ancient Earth

Continuing from the previous page investigating the origins of life on Earth....

Life began to flourish, and the autotrophic organisms had tapped a new niche allowing the biomass of Earth at the time to dramatically increase.

The autotrophs en masse were absorbing carbon and light. The light invariably would always be an available source for synthesising energy, while the carbon was not.

CO2 was constantly being absorbed by these organisms, and after the biological reactions responsible for creating energy in them, oxygen would be released as a by-product.

Aerobic Organisms

This meant that oxygen began to accumulate in the oceans where life existed. This new material would in turn be taken advantage of by the adapting organisms, alias, leading to the creation of aerobic organisms, who used oxygen as a component of their energy creation. This is another example of life diversifying to adapt to its environment and exploit the niches that it could occupy.

This type of evolution continued, where the supply of potential energy making elements and compounds outstretched the requirements of life, therefore organisms continued to adapt to fill all available niches as opposed to competing with one another.

Infraspecific / Intraspecific Competition

Pathogens existed by this time, and were able to leech resources from their single cell hosts, kill them, and move on to the next host after multiplying. On top of this, resources for all organisms alive at the time were being stretched by the increasing population of species' and also the diversity of unique species.

Alias, the exhaustible materials used by species were limited, and they would have to 'fight for their right' to survive.

To do this, natural selection would give them a competitive advantage over other organisms and perhaps relieve stress caused by competition within the species (intraspecific).


One noted event in the origins of life is the emergence of protists. Although these organisms were single celled like all other organisms, they were notably bigger, some being visible to the human eye. This adaptation must have been a selective advantage at the time, either over competitors or taking advantage of an ecological niche.

In fact, the adaptive change is believed to be anatomical. Unlike other organisms, the protists contained cell organelles, which meant that a fundamental difference in the way life operated had arisen in the case of the protists. The occurrence of the protists was so unique that the diversity of them substantiates the Animalia and Plantae classifications, because differentiating characteristics were noted, i.e. the presence of organelles.

Basically, protists are unique because they possess a nucleus which contains the genetic information of the cell and alias the organism. Previous species were more simple in their nature, and did not possess such a complex cellular structure.

The next page investigates how these protists survived in the times, progressing through our evolutionary timeline on Earth.....

The Evolution of Cell Organelles

The previous page noted the emergence of protists, organisms that possessed a distinct cell nucleus and contained organelles within its cell structure. One other important aspect of the up and coming species was the existence of mitochondria, the site of ATP creation in human cells (in ribosomes).

Cell Evolution

The mitochondria is present in both animal and plant cells in today's world, suggesting that the arrival of the mitochondria in the evolutionary chain was slightly before recognisable taxonomical differences between animals and plants.

The mitochondria is unique in the sense that the organelle contains its own DNA, which is derived from its parents. Naturally, as the mitochondria is responsible for the breakdown of organic molecules to release energy (i.e. respiration), this DNA was responsible for the reactions involved to do this.

The remarkable thing about mitochondria is their striking similarity to that of a species of amoeba, where the structure of the two are similar. In this particular species of amoeba, symbiotic bacteria enact what the mitochondria does in more advanced cell structures. The end of this symbiotic relationship no doubt increased parasitism, due to the fact that cells now possessed their own energy supply, they could be exposed and eradicated by the pathogens of the time.

Organelle Diversity

Although geological records for this period are sketchy to say the least, evidence suggests that organelles continued to diversify in this period, further differentiating the taxon that we use today to class them.

Hair like structures called cilia and flagella were developing in some species, allowing them to move with wind and water currents. This general progression and diversification has lead to the range of functions that cell organelles perform in modern organisms.

Organelle Similarity

The most unusual thing about natural is its repetition of a particular characteristic across a broad band of species. Such a situation arises when looking the the development of unicellular organisms at the time.

The organelles developing within these species all have structural similarities in relation to function. As in the example above, the mitochondria on a single cell is very similar to that of an entire species, yet mitochondria are found in almost all forms of organisms that have existed on Earth.

A push-pull relationship is notable in the evolution of these organisms. In one instance, they become more similar, either because the similarity is an advantage or because environmental pressure was forcing natural selection and thus the species to evolve in this way.

On the other hand, organisms were diversifying to occupy previously sterile environments, therefore adapting to better suit their new environment. On the other hand of this, other organisms (as above) would adapt closer to them, due to less competition in the habitat and natural selection favouring a move to this environment

In other words, nature at the time, both parasites and uni-cellular organisms, were more in less in equilibrium, continuing to expand but also moving away/moving closer in relation to other continued to change into the Cambrian Period, over half a billion years ago.

The timeline continues into the Cambrian period on the next page of the timeline...

The Evolutionary Development of Multicellular Organisms

The beginning of the Cambrian era saw a widespread arrival of multi-cellular organisms, particularly in the form of sponges. These species, who inhabited the Earth around half a billion years ago, could grow up to 1 metre across, making this distinctly different from the previous unicellular organisms.

This was the beginning of cell specialisation into tissues, where particular tissues could perform functions to the well-being of the organism at large.

The interesting thing about specialisation at the time is the fact that if you segregated the cells of these organisms, each cell could still live independently. This is a prolonged example in evolution where characteristics within organisms are similar to that of whole organisms, as in the mitochondria example mentioned at the foot of the previous page.

In fact, some multi-cellular species possess organelles that are indistinguishable from some species.

The accumulative induction of advantageous characteristics held by species was obviously being learned by the genome of other organisms, i.e. the permutations and advantages are common and widespread.

Sexual Reproduction

One major event in time is the development of sexual reproduction. Previous species method of reproduction was simply mitosis, repeated cell division which produced new organisms, and exact copy of their ancestors. Of course, mutations and other factors over time changed their genome causing them to evolve.

But with sexual reproduction, genetic information is shared between organisms, meaning that the permutations involved in the long term involving the genome of species greatly increased. This is because of all the variances involved in meiosis meant that the possible genotype of offspring increased, and natural selection could take effect on the unique organisms.

Natural Selection Via Sex

Consider the following

  1. Previous life did not use sex as a means of reproduction
  2. They replicated making exact copies of themselves
  3. Genetic diversity was only increased by mutation and new chemical reactions occurring on Earth making simple proteins
  4. More modern organisms share genetic information by sexual reproduction
  5. 50% of genetic material is taken from each unique parent
  6. The offspring is unique, containing only 50% of genetic material from each parent, plus any change caused by natural selection and mutations
  7. Overall, diversity in the species is increased, causing differences, and thus selective advantages/advantages in comparison to one another within the species, and in relation to their environment

Due to the increased possibilities that life could diversify to with the advent of sex, genetic variation greatly increased, and filled the ecosystems niches to a further extent. Competition for resources with species and against other organisms would be increasing in relation to past times, as populations increased and resources diminished.

In light of natural selection and 'survival of the fittest', organisms would have to fight for their right to survive, and be able to adapt fast enough to their environment to stand the test of time. In light of this predicament to life on Earth, further diversity continued, with the creation of distinct animals and plants arriving on the Earth's surface...discussed more on the next page of the timeline.

Primitive Animals

Previous, incorrect taxonomic classifications deemed organisms to be either animals or plants, in the Plantae or Animalia kingdoms respectively.

This would prove to be incorrect, as there were some organisms that would fit into either/or categories, making the old taxonomic classification unclear.

However, within the Cambrian Era, eukaryote Animalian organisms were thought to be roaming the seas, after further diversifying by natural selection and mutation from the first multi-cellular organisms mentioned on the previous page.

These organisms would have arrived on the planet via evolution around half a billion years ago, gently diversifying over time from their ancestors into new distinct species occupying their unique ecological habitat.

In the grand scheme of things, life was diversifying at an accelerating rate, and thus taxonomists use more precise classifications over the geological periods. The arrival of distinct multi-cellular organisms has led us to distinguish them in Phylum's.

An interesting fact to note is that the mineral composition of modern animals, such as ourselves, is similar to that of these ancient organisms, who originated in the sea. Thus, we can deduce that as animals, and life in general, evolved from the sea.

This period of time gave rise to a multitude of genetic diversity in their respective phylum's (the sub-Taxon of kingdoms) illustrated below;


Worms were some of the first diversifying animals to be found on Earth. Their flattened nature would have helped them in their movement against currents in the aquatic environment.

Remember that movement was a major asset to organisms at this time, thus was of great selective advantage in the long run. Since worms have been around since more or less the beginning of time as far as life is concerned, their genome has been adaptable enough to suit many ecological niches and environments, so much in fact that many species of worm are parasitic in nature. The below Phylum Taxon classification looks at worm species more closely, each divided into their unique Phylum

  1. Phylum Platyhelminthes - Flattened, worm-like animals
  2. Phylum Nematoda - Roundworms
  3. Phylum Annelida - Segmented worms


Molluscs are distinguishable from other animals because of classic characteristics such as the advent of feet, a tongue, an exoskeleton and/or gills. This classification of organisms would either have one or all of these characteristics.

Obviously as molluscs possessed unique characteristics, and thus selective advantages in themselves. The use of feet for example, meant that the sea floor could be combed more closely and efficiently while movement would have been easier with the ability of their feet to claw their way across the ocean and sea floors.


Although these organisms are ancient in nature, they form the largest single living structure on planet Earth today, the Great Barrier Reef off the coast of Australia. These animals evolved developing many offensive characteristics, thus given a Taxon of their own.

Typical characteristics of coelenterates include but not exclusively

  1. Tentacles that are capable of stinging potential danger and also their prey by using neurotoxins
  2. Jelly-like appearance
  3. An alteration of generations. Sometimes these species produces asexually one generation and sexually the next (perhaps suggesting the transition of sex, natural selection favouring one way or the other over time)


Just like all other organisms developing in the Cambrian era, the echinoderms were of a simple nature. One interesting characteristic was their five-part symmetry, noting that the number five is a common number involving projecting areas of the body (i.e. fingers and toes).

'Starfish' are an example of this classification of organisms, which have stood the test of time via the Cambrian Era to today's world half a billion years later. Some people have suggested due to their characteristics that they may be a common ancestor of many all those years ago, but due to the timescales involved, yet again their is inconclusive evidence.

Their 5 star structure of the starfish allows them to prize open clams, sometimes in tandem as a means of nutrition.

These are all examples of the continues evolution of life. These animals were the beginning of many, continuing to diversify in their pursuit of survival. With competition came co-operation, the fight for the right to survive was hotting up, and organisms continued to advance in a sophisticated manner.


There are over two million species of arthropods, who initially arrived on Earth in the middle of the Cambrian period. Naturally, they were more evolved than their ancestors in a variety of ways and thus possessed their own unique characteristics.

Essentially, arthropods are characterised by possessing jointed limbs and an exoskeleton. They are the most successful animal Phylum on the planet, in regards to population size and species diversity. There is thought to be over 2 million types of arthropod in today's world.

The exoskeleton may illustrate what life was like at the time. It is of a defensive, protective nature to possess a shell, thus this suggests that competition was quite fierce in the Cambrian era, both from parasites and potential predators.

The arthropods were also the first taxon of species to exhibit more advanced receptors in the form of eyes (photoreceptors) and the development of various chemoreceptors that could be used in both the external and internal environment. Such developments have naturally been advantageous over time, illustrated by ourselves.

Since the arthropods possessed such desirable features, their survival over the long term is apparent by their genetic diversity, elaborated upon below.


As life originated in the sea, the sea was still a valuable ecological niche to the numerous species of the time. Crustacean means insect of the sea, and is a Subphylum of the Arthropoda Phylum.

Although abundant, the crustaceans remain relatively simple in the grand scheme of life, and thus did not diversify well in comparison to other organisms. Some of the species in this class were able to occupy the freshwater ecosystem over time, though not successful as what could have been. Competition from more adaptive organisms would have been a biotic factor here.

The continued use of feet was evident in these organisms, as a continuation of the organisms mentioned on the previous page of the timeline. The fact that the species' limbs were now jointed, they could move more flexibly and thus had an advantage.

Many crustaceans are herbivores, meaning they obtain food from the consumption of plants. They are of great importance to aquatic ecosystems, and are above species of phytoplankton (micro-scopic plants) in the food chain. This can be related to in the freshwater ecology tutorial investigating food chains and plankton.

Also, many crustacean animals feed on molluscs, the more evolutionary primitive animals mentioned on the previous page.


Including centipedes and millipedes, these species take advantage of the advent of feet and organs assisting movement across the ground.

Since the Myriapods have so many legs, the co-ordinated escape from predators is slow. This has led to them adapting and evolving chemical defences when potential biological danger comes too close.

They also harness the use of chemoreceptors to assist them in their external environment, as well as physiological adaptations to assist them in burrowing into the ground, another method of defence, and also a way of diversifying into ground based environments over time.


Arachnids were one of the first taxon of species to occupy dry land, the first transition from dry land from the life origins of the sea. Due to these bold creatures' actions, their ancestors have successfully realised their species goal of survival, occupying previously sterile, unchallenged environments.

This would have occurred around a quarter of a billion years ago, approximately the same distance in time between the present > then and then > the origin of life. As a side note, it is quite interesting to note that humans begin to occupy space at around the same time scale involving life moving from the sea to land.

Arthropoda - Insecta

To learn more about arthropods, look at the previous page.

The Class Insecta of the Arthropoda Phylum is by far the most successful and diverse taxon on planet Earth. In fact, there are more species of insect than any other species combined. This surely illustrates that insects have particular selective advantages that allow them to take the most advantage of the environment that they live in.

The development of insects was a stamping of authority by animals species on life developing at the time.

Insects possess all the selective advantages of the arthropods mentioned on the previous page plus their own unique advantages with each species of them. Here are some reasons as to why insects enjoyed their continued existence over such a long period of time (beginning over 400 million years ago).

Land Insects

Just like the other arthropods, took the opportunity to occupy dry land, and thus evolved to cater for their new environment. Evolutionary adaptations mapped out in insect species points out the minimum water transpired by the organisms, illustrating their relatively audacious transition from a wet environment to dry land.

Insects also occupy the sea, though face stiffer competition from the continuous evolution that was happening there with other species, creating environmental pressure and an occupational threshold of habitats.


Insects continued to evolve the sense developed by other arthropods and their ancestors, and were capable of interpreting auditory, visual and chemical stimuli.

The Symbiosis of Plants and Insects

Over the evolutionary timeline we have followed, although plants have not been mentioned much, insects were heavily dependent on plant life. Both insects and plants have co-evolved with one another, and if you had removed one of them at any point in history, scores of species would have never existed in today's world.

Insects and plants used one another in a variety of ways

  1. Butterflies undergo a process called metamorphosis, which is a transition from embryonic to adult form of a species. In the case of the butterfly, adults hatch eggs within plants to camouflage them against potential damage and predators who may eat the eggs.
  2. In other cases, insects are herbivores, and thus eat plants as a means of nutrition. In reverse instances, plants like the Venus Fly Trap engulf insects within their defensive mechanisms and kill them
  3. Insects pollinate plants, providing a way for plants to create offspring and successfully pass their genome through the generations

Alias, plants and insects have always had a close relationships in evolution

Social Habits

Some species of insects are capable of communicating with one another. This would be one of the first instances of this in the evolutionary chain, and remarkably happening hundreds of millions of years ago. Bees are an example of a social insect, who perform a waggle dance in front of fellow bees from the same hive to indicate the quality and navigational source of a food supply.

Indeed, insects were an important factor in life's transition from water to land.


While insects and similar types of organism strived to occupy land, the sea was teaming with life aiming to secure their long term survival. As a consequence of this, reproduction occurred and genetic variation increased. This results in the arrival of fish, who were adapting to live in the largest ecosystem on earth, water.

There are over 20 000 species of fish, all of which have diversified over time to aptly occupy a particular habitat. Since aquatic environments vary greatly in regards to its characteristics, fish diversity also varies greatly.

Factor - Temperature

Depending on season, chronological point in time, depth of water and many other factors, temperature will affect how a fish species would occupy or even exist in an ecosystem. An example is some species being better suited to tropical warm waters while others occupy the polar regions of Earth in its present day

Factor - Saltwater / Freshwater

Fish have diversified to occupy saltwater and freshwater in the best way possible. This is further illustrated in the animal water regulation tutorial page. The main reason for this being a significant factor is the effect that salt has on osmoregulation, thus fish have underwent significant anatomical adaptations to occupy the respective environments

Factor - Biotic Relationships

Other species may represent competition, danger, a source of food or provide a symbiotic relationship. Nonetheless, all species are inevitably a factor, and this is indeed the same case for fish. Check out the producer / consumer relationship page in the freshwater ecology tutorial for an elaboration of this relationship between organisms.

Factor - Other Abiotic Factors

Chemical composition, amount of sunlight and numerous other factors would determine the evolutionary lines of fish from the original ancestors.

Factor - Geological Change

Many years ago the Earth was still very unstable, rapid and extreme geological change would have wiped out adapted organisms and promoted change in the more adaptive organisms.

The most primitive fish are invertebrates, of which some still exist today. These would most likely be the first fish to occupy Earth, having diversified from the primitive crustaceans that occupied the sea beforehand. These primitive and relatively unspecialised organisms would have adapted over a long period of time (millions of years) to take into account the factors above.

Also, as competition increased and available habitats decreased, fish would have had to be more aggressive or more co-operative in their nature to survive in the long term. This has led to species like the shark, which is of phenomenal size and represents danger.

Other species have taken a different approach, adopting chemical defences as a means of survival. Others have adapted to occupy very low altitudes, thus avoiding some of the more competitive habitats closer to the water surface.

All in all, fish, alongside the later developing mammals, would successfully dominate the seas. In the future, mammals would occupy the sea from land, but fish done the opposite; they evolved from sea on to land just like the arthropods intended.

Amphibians & Early Reptiles


Many amphibians, like many fish and insects, were vertebrates, and are all under the Subphylum taxon Vertebrata.

Amphibians are typically characterised by their incomplete transition from water to land. They are a class of organism that typically inhabits coastal areas or surrounding aquatic environments.

Obtaining air outside an aquatic environment required species to have suited adaptations, and this was the case of amphibians, many of which contain both gills and lungs for aquatic and above water respiration.

An interesting note to take about amphibians is that the typical life cycle of one involves a transition from water to land, just like the overall transition amphibians took as a collective many years ago

  1. The common frog spawns its eggs with the help of plants in the aquatic environment
  2. These young eggs develop into adults, and head towards land
  3. The adolescent frog moves to land
  4. When reaching sexual maturity, the adult returns to water to spawn eggs, as in step 1

So basically, the entire evolutionary emergence of amphibians is re-acted again and again in each successful generation of amphibian species - like the frog.

Early Reptiles

The amphibians never quite made it on to land, but reptiles did. One of the main reasons for this is the two evolutionary adaptations developed by the common early reptile, waterproof skin a shelled eggs (containing their young). Also, although reptiles were cold blooded just like their amphibian ancestors, they were able to adapt to the warmer, dryer environments found on dry land.

With this sole advantage at hand, they were provided a gateway to further diversify and occupy the habitats of dry land. At the time, it is important to note that other animals and plants were succeeding in occupying land, and thus provided a framework for the early reptiles to exist within.

Competition / Co-Operation

Although reptiles were occupying bold new environments (land / shore and sea), a degree of co-operation and competition would ensure that they would survive and prosper as a collective in the long term. No other type of animal had successfully occupied land at this time.

Through another perspective, biomass on land was low, because not many animals had became adaptive enough to survive on land. With this in hand, many reptiles were herbivores, taking advantage of the hydroseres and other plants available on land or shorelines.

But as these organisms occupied land, when they died, the following would have happened, which would have helped life's chances of fully occupying land

  1. The first reptiles and amphibians to tread land, and die on land would have broken down into simpler organic compounds
  2. This would have enriched the nutrients in the soil, allowing plants to grow, and micro-organisms to exist on a large scale
  3. Organisms who rely on the above would migrate to land, as would any other organism capable of existing in the growing habitat
  4. This continued ecosystem succession would inevitably allow land to support life on a scale similar to that of the sea

And indeed this was true. As the Triassic period came around, around 230 million years ago, the dinosaurs were emerging as the dominant force on land.

The Dinosaurs

Everyone has heard about the dinosaurs. No doubt one of the reasons why is because of their size, another being the fact that they ruled the Earth just like humans do now. In the grand scheme of things, dinosaurs represented a major turn in the evolutionary development of organisms on Earth.

Dinosaurs, the Greek word meaning terrible lizard, were the most advanced reptiles of all time, due to the fact they were allowed to occupy as many ecological niches as they could without much competition in any form. Literally, the sky was the limit.

Although the sky was the limit, it is thought that the original dinosaurs were of very similar nature to that of the early reptiles mentioned on the previous page. Over time, these organisms slowly moved away from moving with 4 feet to 2 feet, essentially bipedal motion.

The Triassic Period

The first distinct dinosaurs occurred within the beginning of the Triassic Period, named after this occurrence. Two distinct types of dinosaurs had evolved, some that were bird-like and some that were reptile-like. Within these two categories, dinosaurs further diversified to become carnivores, herbivores or indeed omnivores. The main fact to heed is that the dinosaurs evolved and diversified at an amazing and effective rate, to the extent that they 'ruled the world'.

The most notable thing about these organisms is their size, they were huge compared to any other organism that had preceded them. This had obvious selective advantage, particular in the form of sheer bulk and power. The herbivores were also tall enough to graze on the top of trees, which were rarely taken advantage of as a source of food by organisms of the time, purely because they could not reach the top of trees.

The Jurassic and Cretaceous Periods

Essentially, the dinosaurs continued to evolve, and in some cases became bigger and stronger, such as Tyrannosaurus Rex, the largest carnivore on Earth at the time. The Jurassic period also witnessed the emergence of the pterodactyl, a pendactyl digitate organism that ruled the skies.

A balanced relationship would have occurred between the dinosaur herbivores and carnivores. The relationship between them, their relationship between other organisms, their relationship with the environment and the movement into new environments would all be under the influence of natural selection in the long run.

While the dinosaurs ruled the Earth, a competitor was emerging, the Mammalia classification of species, of which, man belongs to.

Dinosaur Extinction

No one truly knows how the dinosaurs became extinct, but the fact is they disappeared and a whole host of ecological niches were made available to other organisms, who could harness the resources of these niches due to the absence of competition (and predation) by dinosaurs.

The dinosaurs disappeared around 65 million years ago, with many other land dwelling organisms also dying out around this time. Regardless of what killed off the dinosaurs, it was comprehensive. The general consensus is that a major geological event killed off many of the land dwelling organisms, particularly the larger ones.

This would have caused an overall drop of biomass on land, and therefore 'less food to go round' all the organisms that occupied dry land. Also, many food chain relationships would have been disrupted, causing a gradual breakdown of populations in the long term, sometimes leading to extinction, essentially survival of the fittest.

Summary of Life

Insects, due to their size, were adaptable and already diverse, meaning that at least their short term survival and close relationship with plants (at the bottom of any food chain) was secured

Marine life was still plentiful, and diversifying, while mammals were emerging to be the next dominant force on plant Earth.

Birds were also diversifying, and taking advantage of their proportionately larger body in comparison to insects, alongside their ability to fly.

On the other hand mammals were specialising on land, and trees, which we further investigate on the next page of the timeline below...

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The Ancestors of Mammals

Mammals Ancestors

Humans are mammals, the most successful taxonomic class of organisms to colonise the Earth. The word mammal derives from the Latin meaning of breast, "mamma", where breasts are a consistent trait among mammals in mothers feeding for feeding their young. Coincidentally, the more scientific name for the breast is the mammary gland, which further illustrates the point.

Mammals are a diverse group of organisms, where the majority of them develop their offspring within the uterus of the mother, though exceptions are noted. For example, monotremes lay eggs, like their common ancestors the reptiles and birds.

To further diverse, over time mammals have diversified into the placentals and the marsupials. But before we get into that, first look at the ancestors of the mammals to get a better understanding of how the mammals became dominant in the first place, in accordance with natural selection and geological events.

Ancestors of Mammalia

The taxonomic class Mammalia is within the Vertebrata phylum, which elementarily suggests that the direct ancestors of mammals were vertebrates. This is true of course, as it would have allowed taxonomists to order the species in light of this.

Over three hundred million years ago, when life was beginning to conquer dry land, reptiles had adapted from their ancestors to live on the land, and acquire an ecological niche that otherwise had no competition.

It is believed that a niche of reptiles deemed the paramammals, which have sufficient distinctions between both reptiles and mammals, to suggest that mammals indeed evolved from reptiles.

Circumstantial Change

Although some reptiles were beginning to possess mammal-like features, it was not for another 50 million years that the first distinctive differences were being noticed in species. Land animals were continuing to diversify and occupy new ecological niches and move away from competitive environments. Herbivores soon diversified from the reptiles, while dog-like species were becoming dominant as a competitor to the more reptile-like creatures.

These dog like creatures were beginning to diversify in the land environment, and become a true competitor for land resources, unlike the more water-dependant reptiles. Characteristic changes like cold to warm-blooded, prolonged front teeth, fur and mammary glands helped taxonomists note the difference over time from the transition from reptiles to more mammal like creatures

The Triassic Period (220 million years ago)

While the tussle for resources developed, the mammals remained small and continually changed in various ways of adaptation that allowed them to fill in more land based ecological niches. However, this time on Earth saw the dominance of the dinosaurs, who also derived from the reptiles mentioned above.

Well known to us, dinosaurs continued to dominate and fill a majority of land's major niches for some years to come, but alias, they did not stand the test of time. No one truly knows why the dinosaurs became extinct, but the suggestion of an asteroid hitting Earth would make it plausible to suggest that mammals survived because they were smaller with many species based underground, and also required less energy to survive. This could mean that the mammals were more prepared for such an occurrence, and thus the reason why they survived through the dinosaur extinction.

However, since the dinosaurs were no longer an entity, the mammals now had a huge range of ecological niches to fill, without too much competition stopping them from doing so.

Also, while the dinosaurs ruled on their own accord, true mammals were beginning to develop, exhibiting many of the characteristics you would see in any present day mammal.

The other descendants of the reptiles, Class Aves (birds), were also a dominant force at the time, adopting some dinosaur like aggressive characteristics that were to prove competitive to mammals for some time to come.

Nonetheless, the Class Mammalia of organisms was soon to develop into its own entirety, where all present day mammals are directly descended from. At around 65 million years ago, the first true signs of mammals were to appear.

Early Mammals on Earth

By the time the dinosaurs were extinct 65 million years ago, the worlds land mass had split up into more or less the present day continents as opposed to the Pangaea that was initially inhabited by the first dinosaurs.

Much of this geological change is the factor that moulded the mammalian species of today's planet.

Geological Change and Natural Selection

It is not entirely known what killed off the dinosaurs, but after this time, the Earth's ecosphere was rapidly changing and throwing up a wide range of ecological niches that new adaptive organisms could fit into. This would further accelerate evolution and adaptation by all animals, including mammals.

The most noted difference, as above, was that mammalian species developed in different continents, and although possessed many similar characteristics, they had adapted to suit their own unique environment. Each continent therefore had its own variety of mammalian organisms and their own unique evolutionary chain and direction.

The Miocene Epoch

This epoch is of importance to mammals because it was the time that the super-continent Eurasia (Europe and Asia) collided with Africa, allowing the previously speciated mammals of both continents to diverse into each others ecological niche's, and to an extent, allow offspring to be produced between Eurasian and African species that were sufficiently similar enough still in ancestral terms to still breed with one another.

What was previous a geographical barrier (the ocean) was now a bridge between two continents, which greatly accelerated genetic diversity and competition between species and predator/prey relationships. This survival of the fittest in accordance with Charles Darwin's theory of evolution would greatly help in diversifying species to what would become the species of present day Earth.

More Geological Change

While competition from other organisms (reptiles / fish / birds) was minimal, evolution made its mark on mammal organisms as they continued to evolve and adapt to the ecological niches land offered. In fact, mammals were so good at adapting, that they also began to occupy the air and water in tandem with their main ecological niche, land.

Land provided an area for evolution to continue, where man's distant relatives would have lived and taken advantage of their wild habitats millions upon millions of years ago.

The mammals diversified to the point of speciation, each inhabiting its own ecological niche and exhibiting its own selective advantages.

The Hominids

Although humans in today's world are the most advanced species on the planet, we previously shared the exact same genetic information of other animals in today's world.

But all in all, the human ancestral line involves the hominid family, who diversified from the apes around 6 to 8 million years ago. Since then our evolutionary path has proved to be nothing short of phenomenal

6 to 8 million years ago – The Early Hominids

Since this sort of timescale is massive in terms of the time humans have existed, there is little evidence to back up any solid theories as to the exact date that hominids diversified from their previous fellow primates.

Previous scientific evidence pointed towards Europe being the origin of this occurrence though more recent research suggests that early hominids may have originated in  Africa and migrated South.

The most famous example of evidence supporting this era is the skeleton “Lucy”, found in Ethiopia, in the African continent, where evidence dates her life at about 3 million years ago.

As the only remains from this period are bones, that have survived the test of time, skeletal features have helped us define the evolutionary process between primates and more modern but previous versions of today’s man.

The palaeoanthropologists who discovered such remains like Lucy mapped the subtle evolutionary changes in the skeletal structure of apes and what we define as early hominids, a distant ancestor of our present species.

The Australopithecines

The remarkable evolutionary chain that we follow here involves the australopithecines, Lucy being such a creature. We have little evidence of how these primates actually behaved, but we can distinguish noted differences such as a change in the size of the australopithecines head, pertaining to more like a modern mans.

One deduction made from evidence is that the early hominids were the first of the evolutionary line to move away from the jungle and into the open lands. This could have been a result of increased competition in the jungle, and therefore they diversified to the new location, and then returned to the jungle due to their inability to fit in the ecological niche at present

In summary, three distinct species of the genus Australopithecus existed between 5 and 2 million years ago, all of which exhibited bipedal motion during their existence on Earth.

Australopithecus afarensis

Meaning “southern ape from afar”, this species probably roamed the Earth around 4 million years ago. This name was given due to the discovery of remains in Ethiopia in the early 20th century, where its discoveries in East Africa are restricted to this area, the Afar Triangle in Ethiopia

Australopithecus africanus

Believed to have derived from Australopithecus africanus, this species would have superceded the afarensis species due to its more aptly suited genome. The species remained roughly the same height, though continued to develop long term into a species more similar to man

Australopithecus robustus and Australopithecus boisei

It is thought that these two species are in some way indirectly related to the long term ancestry of modern man.

Early Species of Homo

Around 2 million years ago, a significant change was occurring in the size of brain of the australopithecines. The change in overall structure of the species meant that taxonomists gave an entirely new genus to the species. The species of homo pertains to the more recent ancestral line of modern man, homo meaning the same as, and sapiens pertaining to ‘man’. This certainly seems relevant when looking in hindsight.

The first glimmers of intelligence were beginning to appear in these species that is comparable to modern man. Basic blunt stone tools were beginning to be used, which could be used in a variety of ways in the hominids daily lives. For example, the tools could be used for carving out their prey, or using a stone to smash branches of trees for wood.

This in turn gave them the chance once again to survive out in the open land, as perhaps their distinct ancestors tried but were less prepared and evolved. This competitive advantage in early homo species was a result of natural selection itself, and thus a critical stage in the development of man.

The Homo Species

Over the long term, it looked like homo would supercede any ‘similar model’ of animal due to their unique tool using competitive advantages. At this point in the timeline, Homo habilis was mans link in that time and place, and was typically taller than any of the australopithecines mentioned previously.

Homo erectus

Homo erectus is the Latin meaning for 'upright man'. At around of the Quaternary Period of Phylogenetic classification, the Homo species was beginning to exhibit the characteristics of modern man. No doubt much of this had to do with their superiority over similar organisms in their ecological niche and the newer environments that early man was beginning to occupy.

The brain size of Homo erectus is notably larger than its ancestors, and excavations of the species have been found in parts of China, a long way away from the theoretical ancestral origins of man in Africa (or Europe, see previous).

Homo sapiens

Homo sapiens, meaning wise man was the next movement towards modern man. They existed as early as the Quaternary period (around 1.6 million years ago) and their brains showed increased growth from previous species, and exhibited more intelligence from human records.

The tools being used were becoming more sophisticated, as were the learning and habituation over generations that allowed man to easily adapt to its surroundings. The species as a whole was occupying a diverse range of continents, therefore greatly diversifying our gene pool over a long period of time.

Archaeological finds have also suggested the first use of wooden tools, like spears, through various finds across the Asian, European and African continents.

Homo sapiens neanderthalensis is a subspecies of Homo sapiens well known for its hypothesised common ancestry with man. They arrived on the scene around a quarter of a million years ago, and continued to evolve to around 30 000 BC. Due to the more recent nature of this subspecies, more information has been found out about them, although it is debatable whether Neanderthal man and our own species are one of the same or unique.

The Neanderthals were widespread across both Europe and Asia during this time. From around the time that the Neanderthals were beginning to disappear, the new modern man was offering the newest competitive advantages and ability to adapt and learn. This species is our own, Homo sapiens sapiens.

Homo sapiens sapiens

From 30 000 years ago up until this present day, our own species has exhibited the most advantageous characteristics to adapt and manipulate our environment. The skills accumulated over many generations of our species and continued favouring of advantageous characteristics via natural selection inevitably meant that our species would evolve beyond all recognition in comparison to the other species of the planet.

From this point, the species and its component skills managed to colonise all the main continents of today’s world, bar Antarctica, which still presented conditions unbearable to the species and the technology of the time.

However, more complex tools were being developed, and that has continued over the period of time where we have successfully monitored historical events in our human race.

At this point, human history in the abstract manner truly begins.

Origins of Life Tutorial

1. Precambrian Times (Most Ancient)

All time before life existed, Earth was still a volatile environment, though the origins of life came around during this period.

2. Cambrian Period

The development of multi-cellular organisms, including sponges, primitive animals and various plant life, diversifying from old species

3. Ordovician > Permian Periods

The transition from primitive species to specialised organisms, the increasing of genetic diversity via sex, and the transition from sea to land

4. Triassic / Jurassic Periods

The arrival and dominance of dinosaurs in the air, land and sea, and how they developed from their ancestors and the causalities of their existence

5. Cretaceous Period

The removal of dinosaurs from planet Earth and the emergence of mammals and other more complex organisms

6. Tertiary Period

The evolution of man and further diversification of all species to present date. The transition of Neanderthal man to modern man.

7. Quaternary Period

The evolution of man and further diversification of all species to present date. The transition of Neanderthal man to modern man, and how our dominance of planet Earth has directly shaped the future timeline of planet Earth.