Transitional forms in human evolution

Thank you for starting this thread. It will be educational for me. For example, I just saw this article, but I don’t understand why it is a “game changer.” Maybe someone on the forum can explain it better for me. (Moderators and Chris please feel free to take it down if it is a distraction rather than relevant)

@Christy, can you yourself answer questions 1 and 2 in my original post?

I think the answer is in the last 3 paragraphs of the article.

The finding challenges a previously held belief about how humans evolved.

“We thought A anamensis [MRD] was gradually turning into A afarensis [Lucy] over time,” said Stephanie Melillo of the Max Planck Institute for Evolutionary Anthropology in Germany, a co-author of the Nature studies.

However, MRD reveals the two species co-existed for about 100,000 years, a finding that Haile-Selassie called a “game-changer in our understanding of human evolution during the Pliocene”.

In other words the hypothesis was A anamensis -> A afarensis -> … -> H sapiens. But if they coexisted then we may have lost a transitional fossil. (putting it in terms of this thread)

See here

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Hi! I’m curious what exactly you mean by “on the path.” Are you looking for genetic proof that that individual was a direct ancestor of living creatures? Or that it was the same species of creature as a direct ancestor? What if it was slightly different but could have interbred with a direct ancestor of a living creature? Or, since all of this is going to be nearly impossible to judge based on fossil evidence, if experts in that creature’s anatomy conclude it was “closely related” to a direct ancestor, which is really how you will see it stated all the time because scientists hate to overstate their case?

I would hate to try to answer your second question without clarification from you on what exactly you mean by the phrase “on the path” from your first question.

If you are claiming that species B is transitional between species A and species C then do you mean that species A is ancestral to species B which is ancestral to species C?

Or are you just claiming that it is intermediate in form, e.g. a chimp is a transitional form between a rhesus monkey and a human, but there is no ancestor descendant relationship between them?


As I understand it, it’s not a “path” it’s a tree. So transitional forms refer to ancestors on the tree that are genetically related to one another. I posted tree of human ancestors and an article with an explanation that many of these species coexisted for times. There is no linear progression.

What do you mean by “the terms of this thread.” Transitional fossil means a fossil that has traits common to an ancestor species and a later species.

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Why? Evolution doesn’t require that the ancestor die out immediately.

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I think I am starting to understand what you are getting at.

Well, of course any number of transitional fossils are ‘lost’ if you mean did every generation leave a fossil that we have found. But to say A. anamensis can’t be a transitional fossil anymore if we discover it coexisted with another transitional fossil is like saying I-35 can’t be part of the path from Minnesota to Arkansas because I-35 winds up in Texas, not Arkansas. No species exists at only one point in time any more than it would consist of a single individual.

A rhesus monkey would more closely resemble the common ancestors of monkeys, apes and humans than a human would. That doesn’t make it our ancestor, but it is a great example of what we expect to see in a world where life has evolved.

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Not quite. Here is how the Nature article ends:

In summary, although MRD and other discoveries from Woranso-Mille do not falsify the proposed ancestor–descendant relationship between A. anamensis and A. afarensis, they indicate that A. afarensis may not have evolved from a single ancestral population. Most importantly, MRD shows that despite the widely accepted hypothesis of anagenesis, A. afarensis did not appear as a result of phyletic transformation. It also shows that at least two related hominin species co-existed in eastern Africa around 3.8 Myr ago, further lending support to mid-Pliocene hominin diversity.

I highlighted anagenesis because that is the hypothesis being challenged. Anagenesis is one species slowly evolving into another, while cladogenesis happens when a parent species splits into two distinct lines. Anagenesis already was thought to be rare, so disproving this possible instance deals another blow to a weak idea.

Your mistake, Chris, is wanting to draw a straight line of one species morphing directly into the next and so on and so forth. That is anagenesis, which is theoretically possible but rare.

Beyond that, we’ll have to call in some professional biologists to explain. I’ve reached my limits!


My favorite transitional fossil is the H. sapiens from Jebel Irhoud, Morocco. Dated to 300,000 years ago, it has the small face of a modern human but an elongated skull like Neanderthal and every previous hominin. Our distinctive “globe” didn’t appear for another 100,000 years.

Hublin et al . used a shape-analysis statistical technique to compare the excavated fossils with those of ancient human relatives dated to between 1.8 million and 150,000 years ago, modern H. sapiens fossils from the past 130,000 years and Neanderthals. Facially, Neanderthals and most of the other fossil humans were clearly distinguishable from the Jebel Irhoud specimens, which were most similar to modern H. sapiens . …

The Jebel Irhoud braincases retained some archaic features, such as an elongated shape and low height when compared with the braincases of H. sapiens fossils from within the past 130,000 years. Their external braincase shape was intermediate between that of archaic and more-modern-looking fossils, but was most similar to the late archaic H. sapiens skull from Laetoli in Tanzania and the early modern H. sapiens skulls from Qafzeh in Israel. Their internal braincase shape was distinctive. Perhaps it represents a structure near the beginning of the trajectory that led to the evolution of the globular brain shape characteristic of H. sapiens during the past 130,000 years.


Thanks for the tree, Christy!
Do you think the fossils discovered in Chad, tchadensis and Orrin, from 7 and 6 million years ago, could have occupied the empty leaf cluster in the lower left portion of the tree? I’ve been interested in those finds lately. I’ve read more about tchadensis, and I’ve wondered if it was an early offshoot from the chimp/hominid common ancestor that just died out. Wonder if anyone has opinions. Thanks!


I don’t know, maybe the somebody with more expertise will chime in on that.

And here it is, with my addition showing the path from the putative common ancestor to humans in blue.

All the transitions from ape to human must lie on this blue path so we can see that none of the Australopithecus fossils are on this path, nor are the Ardipithecus or the Paranthropus, or even most of the Homo fossils. In other words the transitional forms in human evolution are non-existent.

Alternatively we could look at this tree.

According to this tree;
Homo sapiens are descended from H. heidelbergensis
which are descended from H. antecessor
which are descended from H. erectus
which are descended from H. habilis
which are descended from Australopithecus garhi
which are descended from A. afarensis (Lucy)
which are descended from A. anamensis
which are descended from Ardipithecus ramidus (Ardi)
which are descended from Orrorin tugenensis
In other words there are several transitional forms in human evolution.

So what are you claiming, are there no transitional forms or are there several which you can name?

They’re probably all just simple monkeys with nothing to do with human evolution. Monkeys which could hike around upright and run and use stone tools and draw pictures in caves, occasionally, but sure, nothing human about any of them.

Nothing that would link chimps to us, as if anatomists hadn’t been perfectly capable of identifying the great apes as our closest living relatives long before any of these fossils were found.

Certainly nothing predicted by Darwin when he said a missing link might be found. What would that even look like?

Certainly not like any of these, that’s for sure!


Hi Chris,

I think you’re getting tripped on definitions. It seems that paleontologists have adopted a new convention in their definitions and illustrations, and we have to “adapt” (hah!) to that convention.

A series of transitional fossils demonstrates the history of transitions that have occurred. A transitional form is not necessarily a direct ancestor of a modern form; it might be, but it might also be the “cousin” of as-yet undiscovered contemporaries that were on the direct line to modernity. Those as-yet undiscovered cousins would be roughly similar to the discovered cousins.

The first time I saw this convention used in a cetacean evolution diagram, it stunned me for a minute. Then I realized that the paleontologists were trying to distinguish between what’s highly certain (the general path of the transitions) and what’s less certain (the exact placement of fossil species on the tree), and I appreciated the effort. Of course, the risk is that the new convention might lead to confusion amongst folks who did not learn this kind of diagram during their education.

The other source of confusion might lie in the way that the Homo group is depicted. I believe that a “group” was chosen rather than branches with leaves because we know from genetic evidence that various members of the different Homo species mated. So it’s not that no transitional form exists; rather, the transitions do not have sharply defined boundaries.

Yeah, I’m having a hard time keeping up. Hopefully this post is mostly in agreement with what anthopologists have figured out. I invite any experts in the field to correct anything I might have missed or misportrayed.



Thanks Chris, that does help.

I saw a while ago a picture arranging antelope in a series of smallest to largest. A series of transitional forms but no ancestor-descendant relationship.

A series of transitional forms by itself means absolutely nothing for support of the theory of evolution. Likewise a series of ancestor-descendants means nothing by itself. It only supports the theory if it is a series of ancestor-descendants that also shows a transition in form.

I am about to leave for a three day retreat so I might not be able to post anything for a few days.

I am claiming that all the species on the tree are considered transitional forms as I understand the term.

How do you understand them?

I just told you. All the species on the tree are transitional forms.

How do you define a transitional form?

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