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Evolutionists say that dinosaurs evolved from a lizardlike ancestor. But that creates a problem.

If lizardlike creatures can evolve into dinosaurs, why did it only happen once? Why didn't lizards evolve into dinosaurs. After all, dinosaurs were very fit to survive.

Just doesn't make sense to me.

Nat

"After all, dinosaurs were very fit to survive."

Umm, not necessarily. Dinosaurs were well equipped to survive in their environment (at least for a long while), but that does not mean that they are necessarily fit to survive in today's environment.

Big and strong does not equal fit.

Wyz_sub10

Also, there would have to be a selective advantage for this. Komodo dragons, for instance, are massive lizards. They are nowhere near dinosaurs, of course, but they're pretty damn big when compared to geckos.

But Komodos are rare and have no real survival advantage over geckos. (Actually, less so)

Remember also that dinosaurs didn't spring up over night. Maybe dinosaur-like creatures will again walk the earth when the environment is more conducive.

You may also ask, why don't we have a new race of humanoids? Why not a new neanderthal ("newanderthal"?) If selective factors are not there, then there is no reason to expect that what happened once will happen again.

[Edited because my proofreading sucks worse than spelling]

[ September 23, 2002: Message edited by: Wyz_sub10 ]

[ September 23, 2002: Message edited by: Wyz_sub10 ]</p>

daemon

Perhaps they are. Evolution is a very gradual process; iguanas don't suddenly have pterodactyls. It may be that, in a few million years, some species of lizards will closely resemble dinosaurs.

Or, they might not--fitness is not so easily determinable, and it is quite obvious they were not fit enough to survive whatever killed them off 65 million years or so ago.

Undercurrent

Might add that "dinosaur" is a very loose category. Dinosaur species routinely arose and went extinct throughout the mesozoic as various environments to which specific species were adapted came and went.

As to the original post question, if you're a species (massive anthropmorphization (sp?) here) and see that Triceretops horridus is doing quite well for itself in its environment and think, "well I'm going to be just like T. horridus" you're bound to fail, because, in addition to the environment that is good for T. horridus, you've got a whole bunch of T. horridusses to compete with who are probably better adapted to that envronment than you are and would beat you out in head-to-head competition.

An "environment" to adapt to is not just the climate, it's also the species that are already there.

m.

Godless Dave

Yep, mammals and birds already fill the niches that were left open when the dinosaurs died off. Now if some massive catastrophe made thousands of species extinct there would be open niches to fill. If the conditions were right new reptile species might evolve to fill them.

MrDarwin

Because:

• the ancestors of the dinosaurs were not lizards
• the ancestors of the dinosaurs have been dead and gone for a long, long time
• current conditions on the earth are unlike those in which dinosaurs evolved

theyeti

Generally speaking, given that evolution is not forward-looking and relies on random mutations as a source of variation, it would be extremely unlikely that a given type of animal would evolve more than once. Closely related species often independently evolve the same trait (known as a parallelism), given that they're genetically almost identical, but it would be next to impossible for more distantly related creatures to evolve identical traits. Sometime distantly related organisms will evolve similar solutions to similar problems (convergent evolution) but their adaptations are always non-homologous. One good example is the body shape of dophins, fish, and icthyosaurs (sp?), which is similar in appearance despite these organisms having very ancient common ancestors. Another is the superficial similarities between a bat's wing and a bird's wing. The desert plants of the American southwest and Australia are another example. But in none of these cases does the same type of organism evolve more than once, because evolution can only work with what's already lying around.

theyeti

KeithHarwood

You are looking at what happened from the wrong end. Yes, dinosaurs evolved from lizard-like ancestors and if you look from the dinosaurs backwards that's what you see. But if you look at it from the ancestors forwards you see that they evolved into dinosaurs, but they also evolved into lizards, crocodiles, marsupials and mammals.
And that's the point. A species doesn't evolve into one other species, it evolves into many as its members try the many different strategies to survival. Either that or the species goes extinct.

Doubting Didymus

In this case, I am not so sure. I think the 'large nasty beast' ecological niche is kind of universal. I compare it to cephalisation. (is that cephalisation, or encephalisation? or something else altogether, my books are elsewhere). Cephalisation is the tendancy for mobile organisms to evolve a 'head', being a collection of sensory organs located at the organisms front. Cephalisation evolves independantly in just about everything, just because it is such a useful thing to have (you can see where you are going, what you are eating, etc).

So there are certain things that just keep popping up in convergent evolution. Things like cephalisation, eyes, and I think, bigness. When the big dinosaurs (most of which were small, by the way), died off, they left a big empty attractive niche. What happened then was kind of like what happens when a big tree falls in the rainforest. All of a sudden the undergrowth hears the starters pistol and there is a huge race to get to the space. So the mammals took over (by coincidence, it could just as easily have been reptiles again). Anyone who saw walking with beasts recently will have seen the kind of mammals that took over.

So I think that, if the big mammals were to die off and only leave reptiles, then we would certainly have giant 'lizards' again. (or giant birds, or some giant thing)