It took 10 to 15 million years after the dinosaurs were wiped out
before modern mammals-including our ancient human ancestors-were able
to diversify and rise to their present-day prominence across the globe,
a landmark new study has found.
The surprise finding overturns
the widely held belief that the ancestors of modern mammals were able
to quickly evolve and spread to fill many of the empty niches left
behind following the mass extinctions of dinosaurs and many other large
animals when a huge asteroid crashed into the Earth about 65 million
years ago.
An international research team reached that conclusion after analysing
the evolutionary links of some 4,500 mammals, creating for the first
time a "supertree" of family relationships between almost all species
of mammal alive today.
Armed with the information about those
relationships, the researchers used DNA data and the fossil record to
estimate diversification rates and work backward to establish when
specific groups of mammals first appeared on Earth.
The study,
which included work by Robin Beck, a PhD student in the UNSW School of
Biological, Earth and Environmental Science, suggests that while some
early mammals may have benefited from the demise of the dinosaurs, many
were on branches of the family tree not closely related to present-day
mammals and died off early on.
They also found that modern
mammal orders, such as primates, rodents, and hoofed animals, did not
diversify until much later, at least 10 to 15 millions years after the
mass extinctions at the boundary between the Cretaceous and Tertiary
periods when dinosaurs went extinct.
This period of
diversification, around 50-55 million years ago, represents the
beginning of the Eocene epoch, and appears to coincide with a peak in
global temperatures known as the Cenozoic thermal maximum. However, the
authors point out that the exact cause of the increased mammalian
biodiversity will require further study.
"Modern mammals went
though an early period of diversification about 93 million years ago,
when the major groups-the superorders and orders-appeared, but after
that the rate dropped and stayed fairly low for the next 40 million
years. It was other groups of mammals, not those we see today, that
took advantage of the extinction of the dinosaurs,' says Mr Beck.
The
research, which appears in the March 29 issue of the journal Nature and
was led by Olaf R.P. Bininda-Emonds of the Technical University of
Munich and Andy Purvis of Imperial College in London, brought together
a diverse group of scientists to produce the first comprehensive
analysis of mammalian biodiversity from about 160 million years ago to
the present day.
The team included contributors came from the
Zoological Society of London, American Museum of Natural History, the
Royal Botanic Gardens, the National Evolutionary Synthesis Center, the
University of British Columbia and the University of Georgia.
Many
paleontologists believed the mass extinction of dinosaurs allowed the
ancestors of modern mammals to flourish and begin the long evolutionary
process culminating in the diverse array of species we see today.
"For
many years, molecular biologists and paleontologists shared different
views about the rise of present-day mammals," said a team member, Ross
MacPhee, a curator in the Division of Vertebrate Zoology at the
American Museum of Natural History.
"Extensive molecular data
indicate that our common mammalian roots have to go back 90 to 100
million years, if not more, but many paleontologists have been dubious
of this claim given the lack of ancestral-looking fossils until about
50 to 55 million years ago. This new work helps reconcile those
differences. Now we know the ancestors of living mammal groups were
there, but in very low numbers."
Supertrees are a kind of
summary of evolutionary history for a large group of organisms
constructed from many, smaller studies for separate groups based on
genetic or physical analysis or both.
They let scientists trace
the evolution of groups of species and map the relationships among
organisms. With the growth of molecular analytic techniques, the
construction and study of supertrees has required ever-greater
computing power, as well as contributions from an increasingly diverse
group of scientists.
The group publishing in Nature includes
bioinformaticians, who gathered and analysed the data needed to
construct the mammalian supertree, as well as palaeontologists and
ecologists, who helped put the information into context.
"The
big question now is what took the ancestors of modern mammals so long
to diversify," said MacPhee. "It's as though they came to the party
after the dinosaurs left, but just hung around while all their distant
relatives were having a good time. Evidently we know very little about
the macroecological mechanisms that play out after mass extinctions."
In
collaboration with other researchers around the world, the team is now
using the supertree to answer a whole range of questions about the
evolution of mammals.
"The supertree itself is really just the
first stage," says Mr Beck. "The information it provides allows us to
look at the overall pattern of mammalian evolution in far greater
detail than before. It has applications in ecology, conservation,
physiology, palaeontology, amongst other fields, and it will also shed
new light on the evolution of our own species-it's a big step
forward.
University of New South Wales. March 2007.
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