To put it another way, when someone says “we have a long series of transitional forms from early hominids to humans and that human evolution seems to be particularly well documented in the fossil record.”, do they mean that we don’t have any fossils from the line of descent from apes to humans?
Not in the least.
You have yet again put forth a “single line of evolution” model, even after I and others have corrected you many times. But you’re a nice guy, so I’ll keep offering corrections until you finally grasp the concept.
There were numerous pathways between lobe-finned fishes and early amphibians. They did not move in lockstep. The process occurred more quickly in some than in others. Are you with me so far?
The appropriate conclusion, then, is that the pathway was traversed more slowly in the Tiktaalik branch than it was in at least one other branch.
We have mountains of fossil evidence showing the transitions from chimp/Homo Sapiens common ancestor all the way to H Sapiens in great detail.
What we cannot show with 100.0% certainty is that any particular Austalopithecus population represented by a particular fossil was definitely on the branch structure that leads all the way to H Sapiens.
Best,
Chris
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To be clear, I believe it is very probable that afarensis is directly on the line of descent, or at least would have hybridized with whatever species was more directly ancestral—that still counts, right?
No. That would be impossible to say with any certainty, and in the case of apes to humans is very unlikely to be true when you consider that usually hybridization within a genus happens. So it’s entirely possible that most of the Homo and Australopithecine species, even ones that dramatically diverged from the line leading to homo sapiens, contributed in some small way to the gene pool.
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Chris, could you clarify for me, is this essentially parallel or convergent evolution, simply happening in one very focused or related part of the branch, as opposed to very disparate branches? In other words, two closely related yet independent fish independently evolved on on entirely separate pathways, although they were closely related both in time and location in the evolutionary tree?
I think that counts. So can we say that you and @Chris_Falter agree that Australopithecus afarensis is probably a transitional fossil on the line of descent between Homo and a more distant ancestor?
I will answer this not as a practicing biologist, but as someone who has read some of the literature. I invite any real biologists to clarify anything I might muddy.
There is no reason to think that DNA copying, copying, or recombination worked any differently 400MYA than they do today. Today we see abundant branching, horizontal transfer, and convergence in biology, so evolutionary biologists expect that to have been occurring as well. Given this background, the almost certain scenario is that many (not just two) pathways of evolution from lobe-finned fishes to early amphibians existed. Branches could have joined as well as split off, some ended, etc. We don’t know all the details, for sure. But enough fossil evidence has been found to strongly support this scenario.
Does that make sense?
Best,
Chris
I think what he means is expressed at berkeley.edu
However, it would imply that Tiktaalik and early tetrapods like Acanthostega have long ghost lineages — a series of ancestors that lived but did not leave behind a fossil record.
Or in other words it is assumed but the evidence is lacking.
By this means any fossil that is conveniently intermediate in form can be called transitional by invoking ghost lineages. This is SCIENCE.
Forgive my ignorance or dullness,then, I’m not following what you mean by “numerous pathways”?
If you mean interbreeding of some sort by various compatible offspring of divergent, but sexually compatible lines, where newly evolved traits arose in one lineage then commingled in order to accomplish the final result, I’d still think of that as “one”, albeit convoluted, pathway.
If you mean “numerous” pathways that were independent, all starting from a similar starting point in fish (perhaps the same exact ancestor), yet arriving at amphibians through different, “separate” pathways (each progressing and proceeding at a different pace, not “moving in lockstep”), then I would call this convergent evolution… perhaps coincident convergent evolution or micro convergent evolution or some such phrase, but convergent nonetheless.
But it is likely I’m simply not understanding the terms you are using. Can you clarify for me exactly what you mean by “numerous pathways between lobe-finned fishes and early amphibians.”
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Of course we are meant to be discussing human evolution but this is an interesting digression in order to try to make sense of what “transitional” means.
Here is another non-biologist who also thinks he may have some general clarity to add - but at risk of needing a mud filter from real biologists myself.
There are probably many super-duper visuals about the “evolutionary bush” to be found in the internet wilds, but instead of trying to find the right one, try this mental picture out.
Imagine a nearly infinitely “fractalized” bush with microscopic “twiggery” that you can keep zooming in to see more of. Except (unlike a real mathematical fractal) you do actually zoom in to a stopping point where the “leaf nodes?” (or ‘twig ends’) actually stop. And that would be at the individual level - like you or me or each presently living creature. As individuals, we are not (yet) some main evolutionary “branch” toward anything, though we definitely have sprouted from a branch that itself comes from a bigger limb that is rooted in a trunk that goes back to a larger trunk … etc. We can’t know in advance which of these tiny twigs will itself eventually become a larger twig or even branch - but even if we could, that branch would not be entirely from you. Because even your own family is not entirely from you. It came from both you and your mate from another family. So there already, two twigs that were already on different twigs “re-merged” together. Likely many of us are probably related to our spouse within a half dozen degrees of cousindom or something. So our bush analogy does break down on this because we have to imagine our tangled bush not having an entirely “star-like” topology, but some of these twigs actually partially merge back together at a later point after they had forked. But anyway, there is no reason to think that this doesn’t happen at “near-speciation” levels where one species becomes almost geographically isolated, but then some interbreeding still occurs - things come back together again. But then in other cases the divergence takes hold to a point where interbreeding is impossible and there truly is a separate branch now that will never again merge back with another main branch. So while our “bush” ends up being quite the tangle at that “microscopic” level, it nevertheless becomes a cleaner “star” topology at a more “zoom-out” level. The likelihood that any set of found fossils represents some cleanly linear genealogy of actual direct descendency would be virtually nill, I’m sure. But instead they will represent the twiggery or leaf nodes of the time from all sorts of things that came from the same branch that something else also was rooted in. Just like if you pick random people out of my entire family tree going half a dozen generations back, the liklihood that you found a line of descendents won’t be good. You probably got a few cousins, aunts, and uncles in there. But that doesn’t mean you couldn’t identify some common family features that might clue you in to our related status to each other.
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If you made me guess at a probability percentage, I would say something like 95% or more, so that really is very or extremely probable. Not, say, 55% “probable.” I would consider it much more sure than Tiktaalik being in the direct line of descent. Tiktaalik shows us a lot, but it’s much farther in the past and hard to be sure without knowing everything else that was alive at that time. That said, I’ve seen mudfish scrambling around out of the water, so tracks aren’t a sure disproof.
Your explanation is perfect. It might be time instead to explore why this community is so confused about such basic things.
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What features do you (and @Chris_Falter) think qualifies Australopithecus afarensis as a transitional fossil?
I wouldn’t try to speak for anyone but myself, but I think it’s so likely because
- Everything we see in the shape of the skeletons we have are intermediate between apes and humans. The legs are more human than ape but still in between both human and ape, the upper body is more ape than human but still in between human and ape. There are no striking derived features such as being very small, very large, having a tail, or specialized ‘nutcracker’ teeth that would point towards not being on the direct path.
- The timing is right. I don’t need to claim that later specimens were on the direct path, although they might be; the earliest specimens currently attributed to Australopithecus afarensis are excellent candidates for belonging to a population of our direct ancestors.
- We have a lot of fossils of this species, so it would have been relatively common. This makes it very likely to have produced descendant species.
- We are not peering all that far back in history compared to, say, Tiktaalik. We have a pretty good idea of what kinds of species existed, and there have been other hominins found, but as far as form goes afarensis seems to have been squarely on the path between ape and human.
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I gave her post a like because I agree 100%. 
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Can you elaborate on that? What makes the legs more human than ape? What makes the upper body more ape? Why are they in between?
Why is the timing right?
Ah, you mean like here
https://evolution.berkeley.edu/evolibrary/images/news/tiktaalik_phylogeny.gif
where Tiktaalik is at a twig end and so not on some evolutionary branch to anything
If you are in any timezone even close to midwestern U.S., then I guess you and I are both early morning people, since I happen to be on now too just as you posted this. [and maybe you are still editing / completing it so I hope I don’t jump the gun with a quick interjection.]
Those tiny and final “twig ends” would only apply to …
1: Presently living organisms,
or
2: Organisms of the past that died before reproducing.
So Tiktaalik is no longer #1 since it is now far in the past. And unless we are given reason to think that #2 applies in its case generally as a species, then we have no warrant for presuming that that Tiktaalik “twig” (at the time) didn’t have its own twiggy tangle woven finally into a larger branch or limb that may still persist with descendants today. But even if that particular species eventually met a dead-end, it would still be like that childless “uncle or aunt” species that is still part of your family tree and still reveals what sort of family history you have had even if they never had any children of their own to make their own branch persist into the present.
[regarding the “four legs” significance - I may not be understanding what you’re getting at. If the diagram you show is accurate, then it would indicate to me that “four leggedness” (or something beginning to evolve toward that) must actually have started earlier than where it’s marked there since the Tiktaalik is considered a tetrapod. But I haven’t researched those details and others who have here probably need to clarify.]
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