Doubting Didymus

Okay, I see your point. The lobe fins might not have the headstart I thought they would, but if the lobefins did take over, they very probably would become four legged tetrapods, as they still have the same little arm/leg bones we have, and would probably not need to revise the srtuctures.

MrDarwin

This is the article you probably want to see, but I couldn’t find a link to it on the web:

DeRobertis, E. M. & Sasai, Y. A common plan for dorsoventral patterning in Bilateria. Nature 380, 37-40 (1996).

Here are a couple of other interesting articles I was able to find in a quick websearch:


<a href="http://www.biochem.wisc.edu/kimble/publications/pdfs/PanganibanEtAl_97.pdf" target="_blank">The origin and evolution of animal appendages</a> (pdf file)

<a href="http://www.embl-heidelberg.de/Conferences/PredocSymposium/Detlev.html" target="_blank">Comparison of early nerve cord development in Bilateria</a>

Doubting Didymus

So the homology we are looking at is fairly minor.

From your second link:

This is suggesting that the 'eye' that is homologous for insects and vertebrates is a single photocell and single pigment cell.

I apologise for my former ranting post, I thought you were making stronger statements than you were. This homologous relationship is traced to a common ancestor with a 2-celled eye, which is hardly an eye at all. I think it is therefore fairly safe to say that, at least when biven a single photreceptor, the development or a complex eye is a universal probability.

I couldn't see anything in your links about encephalisation, which I really think is universal for any forwardly mobile living thing. (probably just because it's the only configuration that makes any sense at all.)

Duvenoy

Interesting.

In my brief absence, it seems the question has changed a bit. Hmm.

But why should evolution begin to repopulate the land with a fish becoming a tetrapod? How about a crustation? There are already numerous species of terrestrial crabs (don't know how many) in various parts of the world, including coastal areas of the US. It would seem simple enough for other species to evolve away from a total dependance upon the sea. While all of the present day land crabs return to the sea to lay eggs, given time, this could change, as it did with our ancient, tetrapod ancestors.

You wanna bignasty, check out a blue crab, a truly ferocious (and tasty), little predator/scavenger/cannible. Then contemplate it warm-blooded and at 500 times the size. Yow!!

It is all but impossible to speculate about life returning to the land because there are just too many possibilties, many (most?) that we have yet to imagine.

If all the large mammals suddenly disappeared, I still think that their niches, including the predatory ones would eventually be filled by evolved, small mammals. There are some dedicated predators in this group and most tend to be prolific breeders as they, themselves are often prey (I'm talking smaller than a skunk, here).

Oh, and I almost forgot: There are also primates no bigger than a rat. What, I wonder, might these become? Or become of them?

This is fun, ain't it?

doov

[ September 28, 2002: Message edited by: Duvenoy ]</p>