How do we decide on the origins of certain strctures?

[mahela007]

When studying evolution, how do scientists decide that certain features in modern organisms were modifications of other organs in their ancestors? For example, how do we know that the antennae and mouthparts of arthropods are actually modified appendages / limbs? As another example, consider feathers. Some books say that feathers are modified reptilian scales. What is the basis for such a conclusion? Feather don't look anything like scales...

[plasmodesmata11]

interestingly enough, there is a lot of controversy surrounding the one instance you mentioned... i remember reading something on scales and feathers. It mentioned something about scutes and how there are different types of scales, like how it was easier, when inhibiting/ repressing certain genes to make feathers grow from where scales would be. That's the answer to the last part at least... sometimes you can repress certain modern genes that reveal more ancestral ones. here... i believe this is what i read
http://www.dinosauria.com/jdp/archie/scutes.htm
hope that helps

[robsabba]

When studying evolution, how do scientists decide that certain features in modern organisms were modifications of other organs in their ancestors? For example, how do we know that the antennae and mouthparts of arthropods are actually modified appendages / limbs? As another example, consider feathers. Some books say that feathers are modified reptilian scales. What is the basis for such a conclusion? Feather don't look anything like scales...

There are mutations that can make segments in insects produce the "wrong' structures, such as antennae where there should be legs, etc. If a more primitive organism has one type of structure in segment 1 and a more advanced species has another, it is likely that the former are modified from the latter.

Developmentaly, feather form from the same cells that scales develop from. In addition, birds have both scales and feathers (as did some dinosaurs)and one can bred birds that have feathers where thee are normally scales. One example of this are chickens which have been bred with feathers on their legs instead of scales.

[Darby]

To a large extent, it's comparison of homologies - antennae aren't modified legs because they don't share an underlying structure, but antennules and legs do. Antennae are homologous to annelid antennae, and so probably predated legs.

Feathers are no longer thought to be modified scales because the two are generated from the skin in very different ways - feathers are thought to have originated as bristles or hairs that sometimes grow from between scales (I think that makes them homologous more to hair than scales).

[skeptic]

Two other lines of evidence.
1. The fossil record. Often a structure in an earlier life form is clearly the same, with modifications, of a later version.

2. Embryology. By following the development of embryos of two different types of animal, it is possible to see a structure in one animal develop into one organ, while in the second animal, it develops into something else. This implies common origin of the two structures.

[mahela007]

Thanks for all the help...