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One of the tenets of Darwin's theory is the survival of the fittest, ie adaptation of features that allow a species to adapt better to its surrounding environment. I am wondering that given the right conditions, is there a possibility of a more "complex" species, say humans, back-evolving into a less "complex" species, say a dog? Is there anything in Darwin's theory or some experimental proof that excludes such a possibility?

EDIT (4/19/12): Thanks for all the responses! I definitely agree with the fact that we are no more complex than dogs or Drosophila, hence the quotes surrounding complex in the question. I learnt a lot from the answers and don't think there is any one answer that is the best, but read together, they drive home the point. Thanks all!

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Since no answer mentions this explicitly, and since it is a very common misunderstanding: humans are not more complex than dogs evolutionarily. The word isn’t even well-defined in this context and no biologist has yet come up with a good enough definition. –  Konrad Rudolph Apr 18 '12 at 23:36
    
@Konrad, I have a feeling that humans, with their speech, math, physics, and understanding of molecular biology, are more sophisticated than dogs and cats. Humans and their associated technologies evolved and developed together, you cannot take technologies away from human development. It is a subtle nuance that part of those human sophistications lies outside of its biology. But it is sophistication nonthe less; and I have to repeat, you cannot take technologies away from human nature, it is part of us. Sophistication way above level of other mammals is present: can other mammals get QM ? –  Andrei Nov 20 '12 at 20:53
    
@Andrei I agree with your (implicit) points about cognition and, what Popper has termed, “world 3”. However, this is strictly outside of the scope of biological evolution (even though it’s a consequence). The “aim” of evolution is simply: to reproduce. This is the only measure of evolutionary fitness that counts. And by this measure, humans are usually not better, and sometimes worse, off than other species. –  Konrad Rudolph Nov 21 '12 at 0:02
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5 Answers

Complexity is a lure. Evolution isn't defined by complexity, but by adaptation to one's environment, as you mentioned.

Evolution is an ongoing, non-deterministic process. There's not one definite evolutionary "path" that brings a unicellular to a modern human being. Nor is there a predictable evolutionary path that brings a dog into becoming a human. You also have to acknowledge that, because evolution is ongoing, the dog of yesterday is not the dog of tomorrow. That applies to all the living.

Is it possible that given the right amount of time and the ideal environment the modern human could evolve into the modern dog?

Yes, it is possible.

Is it probable? No.

The chances are that if humanity were to find itself in such an exotic environment - to which dogs are 100% adapted, but humans 0%, which just shows how unprobable such an environment is - it would become extinct before it has a chance to adapt, much like a lot of other species have done.

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Good answer CHM. It would be a little like running across the field, and then, needing to go back again, stepping in exactly the same footsteps you had taken when coming across the first time. –  shigeta Apr 11 '12 at 20:01
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Good response, but I think it needs to be emphasized that a human evolving into a dog is more than improbable, it is virtually impossible. @shigeta Or hiking a dozen times around the globe, and then retracing each step exactly. –  Preece Apr 17 '12 at 10:37
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There are several strains of mutant cells from humans, mice and various other species that have found themselves a niche as single-celled organisms living in lab dishes, and seem to be thriving there. The well known HeLa human cell line, in particular, appears to have adapted to this new existence well enough to have acquired a reputation as a "weed" that easily contaminates other cell cultures.

Nor is the phenomenon confined to artificial human-created habitats, either — some species of animals, including dogs, hamsters and Tasmanian devils, are susceptible to infection by transmissible cancers, which are essentially unicellular pathogens deriving from mutant cells of the host (or a related) species.

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Interesting, but how does this answer the author's question :/ ? –  CHM Apr 12 '12 at 0:09
    
Well, he asked "is there a possibility of a more "complex" species, say humans, back-evolving into a less "complex" species". I provided some examples where one could say that has, in effect, happened. –  Ilmari Karonen Apr 12 '12 at 0:47
    
So HeLa (and company) cells are considered a species on their own, no longer human? –  CHM Apr 12 '12 at 1:12
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By some people, yes. But even if not, it's hard to argue about the transmissible cancers: the canine transmissible venereal tumor, for example, is certainly a living organism, and it's certainly not a dog. –  Ilmari Karonen Apr 12 '12 at 1:24
    
Very interesting, thanks! –  CHM Apr 12 '12 at 1:26
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Abundant example exist to this when it comes to organs. There are plenty of examples when organs that in the past served some function -- because they evolved to serve this function -- later degenerated or disappeared because environment changed. Examples: vision of species that came to live underground, wings/ability to fly of non-flying birds (penguins, ostriches etc), legs of snakes, some organs of sea mammals.

We know what adaptations mammal species develop to life in the sea. However, these mammals do not turn to cold-blooded fish with scales and gills. They take different evoluionary path for adaptation to sea life. In particular, they remain warm blooded, milk-feeding, and retain lungs. I do not think it is possible that lungs back-evolve into gils, but it is interesting question which mechanisms a mammal like dolphin would evolve if it needed to live all the time under the surface of the sea. It would be matter of inventiveness of mutations in the face of new (old :-) conditions not the issue of back-evolution.

I do not think it was ever shown that genome has mechanism in which instead of mutations, "old" genetic trails (like gils) would be used. I think such mechanism does not exist, and in this sense, the answer to your question is "no".

However, if chain of mutations exist that brings a species back to s atate resembling "old" state, this path might be taken. (degeneration of ograns). In this sense, the answer is "yes".

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It should not have been "survival of the fittest" but rather "survival by the most persistant." Traits evolved by natural selection are not required to be optimal or even adaptive.

In response to your question, consider the case of the evolution of thermal reaction norms. These mathematical representations quantify an organism's performance across a range of environmental temperatures. Across the diversity of life, you'll find that some organisms have evolved narrow thermal tolerance windows while others have evolved broad thermal tolerance windows. These specialist vs generalist strategies have benefits and constraints that depend on environmental and ecosystem stability.

So I think a way of answering your question would be to consider the evolution of thermal reaction norms. Are there derived taxa that have switched between specialist vs generalist thermal preferences compared to their sister clades? I'm sure the answer is yes -- but perhaps check out the papers by Kingsolver (Duke) for a more thorough answer!

Also, consider this: As humans, we are just as much evolved as a fruit fly or a paramecium. Since the generation time is much less for fruit flies and paramecia, you might even say they are more evolved than we are.

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It's always helpful to directly link to papers you mention :) –  Rory M Apr 17 '12 at 7:52
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Just to clarify, as one might read your question as if humans had evolved from dogs - humans and dogs have a common ancestor, one is not a direct descendant of the other.

In terms of evolution running backwards, it has actually been experimentally tested in microorganisms, where you can check the phenotype (characteristics) of an organism over hundreds and thousands of generations. One example that comes to mind is Joe Thornton's research where he reconstructed a gene for an ancient protein and compared it to the more modern versions. They found out that the key mutations that modified the protein sequence could not be simply changed back to obtain the more ancient form, because in the meantime many neutral mutations had accumulated in that protein and while they did not change the protein sequence, they interacted with the protein-changing mutations and prevented the de-evolution of the protein.

The paper I have in mind (there are many more on this) is: Bridgham, Jamie T, Eric A Ortlund, and Joseph W Thornton. 2009. “An Epistatic Ratchet Constrains the Direction of Glucocorticoid Receptor Evolution.” Nature 461 (7263) (September 24): 515–519. doi:10.1038/nature08249.

On the other hand, in a sort of more esoteric example (as it is not directly de-evolution), blind cavefish have multiple mutations (and sometimes in multiple genes) that cumulatively result in loss of vision. Let's then say you have population of the fish that have sight-losing mutations in gene A and another population with mutations in gene B. Both populations are blind, but if you could cross the fish from the two populations, the eye-loss mutations in genes A and B would be substituted by 'correct' versions from the other population and thus both fish would be able to see again. While not de-evolution in itself, it shows that for relatively simple changes that were not influenced by other changes in the genome, you can rewind the clock and get the long-lost phenotypes, even like the sight.

See here: Borowsky, Richard. 2008. “Restoring Sight in Blind Cavefish.” Current Biology : CB 18 (1) (January 8): R23–4. doi:10.1016/j.cub.2007.11.023. (this is a description of the research, not the actual paper; but it links to it).

Both these examples are super-cool (IMHO), and in my collection of all-time favorite evolutionary stories. But practically speaking, what you're asking for (from humans and dogs to their most common ancestor) is not possible. Too many variables (both genetic and environmental and stochastic) to control. The papers are behind the paywall, but if you want them, send me an email.

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@myself The above blind fish example assumes that the mutations in genes A and B are recessive (only one correct copy of the gene is necessary for correct function) –  yotiao Apr 17 '12 at 8:48
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