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If a tribe of homo evolves into a wild animal that doesn't resemble at all a hominid, is there any chance for this animal to evolve back to the original humanoid shape through atavism/other means?

Like homo>large herbivore quadruped>homo.

Is it possible for this to happen in less than 1 billion of years?and what would make it happen?

And in case the ''large herbivore quadruped'' splits in 700 subspecies how likely is for at least 4 of them to become again something that resembles the appearance of a hominid?

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    $\begingroup$ I don't think evolution works the way you think it works. But I think I can make your scenario happen with a little genetic shenanigans. Posting answer now. $\endgroup$ – Mindwin Aug 19 '16 at 19:30
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I agree with @TrEs-2b basically. I also do not know of any examples of complete atavism but you can get pretty close. You asked if the wild-animal could evolve back into a humanoid "shape," and that is by definition what atavism is: phenotype not genotype.

Fish to Mammal to Sea-Mammal to Land-Mammal

Here's a non-homo example I think fits your idea. Whales and other sea mammals evolved from land mammals (which is where mammals first evolved, on land). Land animals (tetrapods) evolved from lobe-finned fishes, which are bony fishes. Whales look a lot like fishes but there are significant differences. Whales have lungs not gills and can't breathe underwater. Whales have hips and thigh bones, but they are small and don't grown into full rear limbs. Whales tailfins are horizontal, fish are vertical. So whales did not actually revert back to fish, but they sure look a lot like them.

We could go one step further (for the sake of your question): sea mammals could start making forays back onto land! Now, being mammals, they have already solved a lot of the problems that the first land-dwellers had to overcome (like skin and eggs drying out). So for a sea mammal to become a re-evolved land mammal would actually be a bit easier, in theory!

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  • $\begingroup$ Let me clarify that for "atavism" to occur I believe the organism has to retain the DNA for that ancient trait and re-express it. Also, usually the trait is just a single trait among many. However, at the same time, the organism may continue to evolve and might by happenstance independently evolve the same traits that its ancestors had. In this latter case, the DNA was lost but then a very similar set of DNA happened to evolve. The fish/mammal example is not atavism. However, I think it's a feasible way for the storyteller to tell some evolutionary tale. $\endgroup$ – Xplodotron Aug 19 '16 at 17:32
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Theoretically there is nothing that will stop a species from evolving into a previous form, but to my knowledge there are no examples of this.

In the span of a billion years a lot can happen, if the environment changes so that the species needs to adapt to it, they will evolve to meet the new environments requirements. Then if the environment changes back to a previous version, the species that live there will either move or evolve back.

Keep in mind that the chances they will do this are extremely unlikely and even if it does happen, do not expect them to look 100% like their similar looking ancestor. Imagine the differences between foxes and dogs for help on this (just an appearance comparison).

For your 700 subspecies, chances are that if the environment changes back, that many will go extinct, some will leave and some will change. 4/700 is almost .5% so it is in the realm of reasonable doubt that .5% of the subspecies will evolve back into a previous form if the environment allows it.

So the answer is yes, but with a big ol' asterisk.

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    $\begingroup$ It's important to remember that most of the evolutionary changes of mammals so far have happened in the last 250 million years. A billion is enough time for almost anything to happen. A book well worth reading if you want to write about future human evolution is Olaf Stapledon's Last and First Men. It's somewhat dated, but it has a lot of ideas worth borrowing. $\endgroup$ – John Dallman Aug 19 '16 at 17:41
  • $\begingroup$ @JohnDallman true, primates have only been around for 50 million $\endgroup$ – TrEs-2b Aug 19 '16 at 17:48
  • $\begingroup$ @JohnDallman I was going to mention "Last and First Men" too but you beat me to it. First thought in my head when I saw the question. Good read. Maybe a bit too long/tedious for some people, but I really like it. $\endgroup$ – Tonny Aug 20 '16 at 11:21
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Extreamly unlikely, and probably not the way you are thinking. Evolution works on small changes, primates and ungulates have no recent common ancestors so you group would generally have to move toward a form similar to a common ancestor, a shrew or mole like critter, then back along the other branch up to large herbivore.

Possibly in less than a billion years, but evolution doesn't happen in a vacuum, so to speak, everything else would still be around, also evolving. What would drive the evolution of the primates to such a radical, lengthy change process? Why would there only be survival opportunities along that path?

The part about atavism is totally off base, atavism is a trait (a single trait) that appears to be from an older "version of the species. The change you are talking about is so profound the only real relationship is that they are both still placental mammals.

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  • $\begingroup$ I wasn't thinking about ungulates.. but something more like giant gorillas $\endgroup$ – Charon Aug 19 '16 at 17:17
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    $\begingroup$ You said "large herbivore quadruped" that doesn't fit gorillas. $\endgroup$ – Seeds Aug 19 '16 at 17:20
  • $\begingroup$ Additionally, you said "wild animal that doesn't resemble at all a hominid" Gorillas are hominids. $\endgroup$ – Seeds Aug 19 '16 at 19:26
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If by evolution you mean genetic modifications over a long timespan, then there is one way to make the scenario work:

  • Genes can be activated or deactivated depending on several other factors. The field of epigenetics is recent, so there is plenty material for soft sci-fi to work with.

  • Human genome has tons of dormant DNA. Most is supposed to be viral/retroviral-transcripted DNA from past diseases, but we still don't know what is out there.

  • Modern Homo sapiens Embryonic development shows several features of past organisms. One example is that human embyros have gill slits. Further reading here.

So we have hard evidence that features from ancestor species are still present, that they can be turned on and off depending on external factors.

So in your scenario, something happened to change humans back to quadrupeds. I'll name the main species Homo quadrupes. But the genetic mutations responsible for such change did not overwrite the biped genes, only supressed them.

After a billion or so years, in some Homo quadrupes tribes, a genetic mutation caused biped offspring. What really happened is that the dormant genes were reactivated.

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