Doubting Didymus

Well sure, but the large mammals we have today overtook the large mammals of a couple million years ago, which overtook the ones previous to that.

'walking with beasts' fans will have noted that every episode contained a brand new set of mammals, the old set either having become something else or giong extinct.

Same story for the dinos. new sets of biglizards replaced old ones and so on for millions of years. They also survived several mass extinction events. Large things just keep coming back no matter what, and the funny looking mammals will just replace the old set of mammals. Big mammals are very likely to stick around for a looong time yet, mass extinctions or no extinctions.

Duvenoy

"The ultimate fate of all sepcies is extinction."

I don't remember who said that, but I have never seen a reason to doubt it.

Further, it is not possible to predict which species might become dominant due to being unable to predict what conditions will prevail in the distant future. However, lizards have been so successful under such a varity of conditions that I think they will retain their basic form. I think some small mammals and/or birds would evolve to move into the nichs left by the larger ones. Again, that's just, "I think...." not any sort of a prediction.

As for whales coming ashore, I think that they've gone too far down the marine road. They might certainly evolve into something else, but IMO it would be aquatic.

Evolution can indeed do a great deal with very little, given time. But would it do the fish/tetrapod trick again if the opportunity arose? Or would it send something entirely different on to the land? Would it again make a sentient animal? And if so, why not more than one?

I never underestimate evolution.

doov

[ September 26, 2002: Message edited by: Duvenoy ]</p>

MrDarwin

I'm sure somebody would have said the same of fish, 400 million years ago.

Doubting Didymus

'large mammals' is not a species. I think that big mammals will just keep coming back, or be replaced by another collection of big species. In my opinion, bignastiness is just too successful for a species to ignore it. Just like developing a head, and developing eyes, bigness happens to anything that has the chance.

And the ultimate fate of all species is not extinction. Every species alive today is the end result of countless ancestral species that never ever went extinct. Homo erectus did not go extinct, he evolved into sapiens. I for one do not count 'evolving into something else' to be 'extinction', even though we are bound to look at the fossil record and say 'that is not here anymore, therefore it is extinct'.

Technically this means that at least some of our living species are not fated for 'extinction', but to descend until the end of life itself. Forgive me if I am optimistic enough to include our own species in that number.

I think so too, but seeing as we are speculating, what if we lost all mammals, and all birds, but kept the lizards? I feel confident saying that under those conditions, I guarantee that 'dinosaurs' (big 'terrible reptiles') will roam the earth once more.

Too far down the marine road? Was a certain ancient marine invertabrate 'too' marine to evolve into a human? You had better hope not! If a lobe finned fish can do it, so can a dolphin or whale. In fact they are a lot more qualified, having been there before.

You are correct that something entirely different would probably happen, given that kind of evolutionary 'reset'.

However, I think that, if the lobe finned fish were left, they would surely take the land before anything else could. I think lobe finned fish or coelecanths (sp?) would certainly become 'tetrapods' again, with four legs and the modern bone stucture (femurs in the right place, etc).

The only way you can predict evolution this way is with knowledge of large trends. Because of this, we can confidently predict that new organisms will have heads and eyes, that any given animal group would probably become huge if given the chance, and that lobe fins would probably colonise the land again if given the chance.

We can not make predictions about sentience, as we have no way of knowing if we are a fluke, or an obvious kind of trick.

RBH

DD wrote And I'd suggest the jury's still out on whether intelligence is an adaptive trait in the current selective environment. We (homo saps) haven't lasted long enough for that conclusion to be even hinted at yet.

RBH

Doubting Didymus

Hmm.

Nonetheless it must have been highly advantagous at at least some point, and for long enough to actually give us the intelligence we have.

Oh yes! there is also the sexual selection theory, which suggests that displays of intelligence may at one time have been a sign of very good genes, the best for a prospective partners children, and so the genes for preferring intelligence in partners would attain a positive feedback loop, increasing exponentially both human intelligence and a preference for things that signify it. (a sense of humor or musicality would be dead giveaways).

There is one way that intelligence could become greater than we need it to be.

tronvillain

Duvenoy:
Ah, but upright posture did evolve from sprawling posture, so I fail to see why it happening again should be considered such an unlikely possibility.

MrDarwin

and


I meant to take issue with this earlier, but got sidetracked. Until quite recently, it was thought that eyes had evolved from scratch several times completely independently in several different phyla. However, recent genetic work is suggesting that the common ancestor of just about all bilateria--that is, pretty much all metazoans after sponges and jellyfish--probably had a head and primitive eyes. So I would hesitate to say that encephalization (is that the right word?) or the evolution of eyes have occurred independently in several different lineages. In fact, these things may have originated only once, meaning either that they were improbable events, or events that were so advantageous that they (a) were rarely lost in any subsequent lineages, and/or (b) they fueled the phylogenetic explosion of more complex metazoans.

Now, the complex eyes of vertebrates and cephalopods may be examples of convergent evolution, but they were both starting from a point of some kind of visual system, and there are only so many different ways to invent a complex eye.

[ September 26, 2002: Message edited by: MrDarwin ]</p>

MrDarwin

You're making the assumption that it was something about their fin/leg structure that led them to become tetrapods in the first place. Maybe so, but maybe it's the other way around, and something else entirely that had already set them down the path to becoming tetrapods also made it necessary to develop better, stronger, more limb-like fins.

You're also assuming that modern lobe-finned fish retain the same set of specializations, or would be exposed to the same environmental conditions, as those that led their cousins down another path millions of years ago. But why would something like the coelacanth (or lungfish, for that matter) have any advantage, when there are already fish like snakeheads and certain catfish that can survive long periods on land, and can even "walk" from one body of water to another. And then there are fish like mudskippers, that spend most of their time on land, to the point of almost looking like little amphibians.

Doubting Didymus

First, I want to see those studies! can you provide links or refs please.

Second, how could this possibly explain non-homologous heads and eyes? How on earth could a compound eye, (which would have started with a convex sheet of photocells), and a lens eye, (which would have started with a concave sheet) be descended from the same 'eye'? What would this eye have been like?

How can organisms that use totally different configurations of photocells (i.e. nerves in front of the retina compared to nerves at the back) have come from the same eye? The only 'eye' I can think of that could possibly be a common ancestor for all modern eyes would be a totally flat sheet of photocells, as per enchinoderms.

Speaking of echinoderms, many of them have no head, and yet they are supposed to share a common ancestor with insects, that has a head? This would suggest that echinoderms lost their heads at some point.

Are protostome heads and deutorostome heads somehow supposed to be homologous?

I'm sorry to have ranted, but it seems ver unlikely that arthropods, for example, have heads and eyes descended from the same source as the chordates.

Edit: I seem to have ranted unneccesarily.

Okay, but that visual system was almost certainly not an 'eye' as we know it, but a flat sheet of photocells.

[ September 26, 2002: Message edited by: Doubting Didymus ]</p>