The Dawn of Our Own Genus: The Rise of Early Homo | The BioLogos Forum

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It is assumed that the origins of our genus, Homo, is derived from a species of Australopithecus, the “southern ape man from Africa,” discovered by Raymond Dart in the early part of the last century. Since the discovery of that find, the number of australopithecine species has reached ten, all dating to between 3.8 and 1.1 million years before the present. Bridging the gap between any of these species and our own line has been difficult, however. This owes in large part to the spotty paleontological record from the period of time between 2.0 and 3.0 million years ago, the period in which this transition is thought to have taken place.

Consequently, the search for our own beginnings has been a study in frustration, where vast stretches of time go unremarked with fossil evidence. Beginning around 2.0 million years ago, early Homo simply appears on the landscape, with no easily discernible antecedents. Further, there is rapid diversification of this form into what have come to be known as Homo habilis, Homo rudolfensis and Homo ergaster. These display a mix of large and small faces and brains. [1] This has led to a confused understanding of where these hominins fit in the picture of human evolution. Now, with one fossil find, much light has been shed on this transition.

In January of 2013, a jaw fragment with teeth, catalog number LD 350-1, was unearthed in the arid region of Ledi-Geraru, in the Afar Triangle, which is part of the Horn of Africa and is known for its rich early hominin deposits. Radiometrically dated to between 2.75 and 2.8 million years ago, this find fills an important gap in the knowledge of our hominin ancestors and pushes the first known appearance of our own line back 400 thousand years.

Two papers in Science and one in Nature outline why this find is so important to the study of early Homo. These studies also point to the recent revolution in paleontology, in which the focus has shifted from the identification of transitional forms to transitional features in the fossil record. With this conceptual shift, transitional features are found in many different taxa and can be easily recognized. Then the question does not become “Is this a transitional form?” but rather “Does this form display transitional features?” This has allowed paleontologists to more clearly understand evolutionary relationships between related taxa.

Using this framework, an examination of the new find reveals that it shares traits with some current examples of early Homo to the exclusion of all but one australopithecine, Au. Afarensis, and shares some traits with that hominin. Those it shares with Au. Afarensis (the species of the famous Lucy fossil), include a shortened, smaller region where the chin would be and an overall smaller jaw, relative to later species. Traits that are derived relative to Lucy in the direction of early Homo are present in the form of the lack of a hollow spot behind the premolars, which characterizes all Au. Afarensis finds, the overall tooth and crown shape and the wear pattern of the teeth. As the describers note: “In the majority of traits that distinguish it from this species, LD 350-1 presents morphology that we interpret as transitional between Australopithecus and Homo”.

These traits also place LD-350 further along the line leading to modern humans relative to other australopithecines. In all of the analyses involving tooth and mandibular size and shape, the LD-350 specimen plots away from all other australopithecines, who tend to have large cheek teeth and strong jaws. This is, perhaps one of the most important tentative conclusions that this find presents: with its constellation of traits and its early date, it relegates all of the australopithecines, save Au. afarensis, to a side branch of hominin prehistory. As such, they represent their own “human-like” evolutionary trajectories and, like so many other species in the fossil record, they all suffer the ignominious fate of extinction. Even Au. sediba, with its modern-looking hand, human-like spine and possible association with stone tools, because of its late date of 1.9 million years BP, cannot be ancestral to early Homo. This raises tantalizing questions regarding the possible cognitive advancements in late surviving australopithecines, which would have run parallel to those of our own genus. As the rich pageantry of human prehistory grows, hopefully these and other questions will have answers.

  1. DiMaggio, E. N., Campisano, C. J., Rowan, J., Dupont-Nivet, G., Deino, A. L., Bibi, F., . . . Arrowsmith, J. R. (2015). Late Pliocene fossiliferous sedimentary record and the environmental context of early Homo from Afar, Ethiopia. Science. doi: 10.1126/science.aaa1415
  2. Gibbons, A. (2015). Deep roots for the genus Homo. Science, 347(6226), 1056-1057. doi: 10.1126/science.347.6226.1056-b
  3. Hofmann, J. (2014). A Tale of Two Crocoducks: Creationist Misuses of Molecular Evolution. Science & Education, 23(10), 2095-2117. doi: 10.1007/s11191-014-9696-8
  4. Spoor, F., Gunz, P., Neubauer, S., Stelzer, S., Scott, N., Kwekason, A., & Dean, M. C. (2015). Reconstructed Homo habilis type OH 7 suggests deep-rooted species diversity in early Homo. Nature, 519(7541), 83-86. doi: 10.1038/nature14224
  5. Spoor, F., Gunz, P., Neubauer, S., Stelzer, S., Scott, N., Kwekason, A., & Dean, M. C. (2015). Reconstructed Homo habilis type OH 7 suggests deep-rooted species diversity in early Homo. Nature, 519(7541), 83-86. doi: 10.1038/nature14224
This is a companion discussion topic for the original entry at https://biologos.org/blog/the-dawn-of-our-own-genus-the-rise-of-early-homo
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The author (@Jimpithecus) is available to respond to comments and questions.

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Fosssils form when some kind of disaster buries an organism before scavengers or decay can destroy it. What kind of disaster (mudslide or volcano) preserved these early hominids in Africa? How far apart in time are the fossils that have been found? I would think that any trait heading towards homo sapiens would have spread rapidly, so that it would seem to have appeared instantly in the fossil record.

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There might have been any number of things that happened. This area was not arid at the time, but was grassland. It could have been attacked and killed. The find was not close to the top of a volcanic tuff so it not likely he/she was buried in an eruption but was covered up quickly in some way. How the find is dated is that it is found between two volcanic tuffs that could be securely dated.

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The huge variety of skulls, teeth, postures, skull sizes and other characteristics in the human race today leave me completely unconvinced that these fossils always represent different species. I agree with the author that the search for beginnings is a study in frustration, probably because the overlying general paradigm is wrong.

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One of the issues in human palaeontology is that we are dealing with truncated time periods, so it is hard to know how much variation ought to be present in any given species. In the search for the ancestry of birds, on the other hand, you are dealing with tens of millions of years, so it is pretty much a given that you are dealing with different species or even genera. The first early birds show up in the Triassic. In human palaeo, you have different character traits that show up at different times. How do you address those differences? Simpson proposed the evolutionary species model, in which anagenetic change occurred in lineages which led to new species. The systematics revolution flies in the face of that. The Simpsonian model is species-oriented, while the systematics methods is trait-oriented. This is still being worked out in human palaeo.

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Ultimately, I think the systematics model is the best one simply because transitional “forms” tend to be mosaic in their morphology. Some species will have more derived traits than others, or will have traits derived in a different direction. If you focus on the “species,” you become tied to a unilineal model, which may not accurately describe the evolutionary history of a group of related species.

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James, I detect some errors in logic in your reply, hope you don’t mind. Variation might be correlated to time, maybe, but it is not determined by time, and thus we cannot say how much variation"ought" to be present. One article I read directly stated that some genetic characteristics in the mouse to man comparison appeared to be on a slow time clock while others were on a fast pace. If you accept this paradigm, then it is clear that time does not determine the variation. In the same way, time does not provide a “given” that we are dealing with different species or genera. Some species and genera that are found in the early fossil record are still present today. It is conceivable that they diverged into new species, while still maintaining the originals, but this means that time does not define this process (assuming you accept the process). Regardless of what model one uses, incorrect assumptions about drivers of the model will result in incorrect conclusions. And models are difficult to verify without genetic evidence, since there is also a wide variety of homologies within identical or near-identical genetic conditions. And, on the other hand, there are also similar macro-homologies with widely differing genetics. (It is not only structure, but also growth process that is part of the identity of an organ or a species.)

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I did not mean to imply that there is a necessary time component. In fact, that is what sets systematics apart from phenetics: that there is no necessary time component. This is also why it is dang near impossible to draw unilineal relationships between forms. The best we can do is identify sister taxa. The problem, in human palaeo is that it seems like we are observing a football field that has a large marching band on it and trying to figure out what the band is doing using a pair of binoculars. The best we can do is focus on three or four people at a time. That, perhaps, is part and parcel of palaeontology as a whole but it is much more acute in human palaeontology. We know, based on radiometric dates, that some forms predate others so the best we can do is see which traits are derived on form B relative to form A and try to draw taxonomic trees based on that. That is why this find so upset the apple cart. It is a full 400k years earlier than any other specimen of Homo. Time intruded on phylogeny.

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I agree that the best we can likely do is identify similar taxa, or all taxa as to their unique or similar characteristics. Based on radiometric dates, we do not know that some forms predate others. The reason is we only know which species were there that we have fossils for, either because they have not yet been discovered, or because the species did not submit to fossil forming conditions. We do not know which species were there for which fossils have not been found. We cannot prove lack of species based on lack of fossils; there are several examples that would contradict this assumption, such as eras in the geologic record that have no fossils of the Coelecanth fish, or other ancient species that are still alive today. I recently also was informed that Europe contained fossils of kangaroos, while Australia does not, which was unexpected. For this reason, that absence of fossils does not prove absence of species, we cannot see which traits from B derived from A, since we do not know the divergence. It is not unusual to find shifts in the supposed philogeny of humans, and we can expect to find more in the future that will upset the applecart of opinions. Ancestors and intermediaries for “early” tetrapods are also constantly being revised for the same reason.

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You are correct that we cannot know for sure. There might be a situation where we discover a very early, less derived form of Au. sediba, which might result in some taxonomic trees then showing it as a possible ancestor but right now, the trees are based on what we do know and what we do know is that, at 1.9 mya, LD 350 is derived in the direction of Homo and possesses basal australopithecine traits. Au. sediba, on the other hand, possesses derived traits away from Au. afarensis and shares some traits with Au. africanus to the exclusion of Au. afarensis. This makes it very hard to consider as a possible early Homo ancestor, based on what we know now. The problem is that there seems to have been quite a number of forms running around in a very compressed time-period. Based on the trait polarities, the authors are suggesting strongly that Au. afarensis gave rise to both early Homo and the later australopithecines, who proliferated and radiated throughout Africa.

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From the article - “Beginning around 2.0 million years ago, early Homo simply appears on the landscape, with no easily discernible antecedents.” One jaw fragment with some teeth is going to change this? That’s not what I call science.

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The fact that there are no easily discernible antecedents does not mean that there are no antecedents. Science is trying to determine what happened. It is not anti-science to write that we just aren’t sure what happened but are trying to figure it out.

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You are absolutely right, James. No discernible antecedents does not mean there are not indiscernible ones. Just as a lack of fossils of any kind does not mean an absence of species of any kind. It also, does not mean that there are antecedents either. And of course scientists are trying to figure out what happened, and are not sure. You have spoken truth. Pity plain truth does not rear its head more often.

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Consider the following example. Neil Shubin knew that there were lobe-finned fish in the middle Devonian and early tetrapods in the latest Devonian. So he went searching in late Devonian sediments for a critter derived in the direction of early tetrapods. Lo and behold: Tiktaalik rosae. The transition had to have happened at that point, so they used the predictive nature of evolutionary theory to find what they suspected would be there.

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Problem is that fossil tetrapod footprints have been found in Poland dated at 395 million years… well before (30 million yrs before) the specimen called tiktaalik is dated. And of course, fossil amphibian-like footprints have also been found in the Grand Canyon, dated at about 150 million years earlier (540 mill yr BP). The predictive nature of evolution has failed once again.

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You are still thinking unilineally. In the example that I gave, Tiktaalik is one of a bunch of different forms that have a mosaic of traits that are transitional between lobe-finned fish and tetrapods. Remember, evolutionary theory cannot determine direct ancestors, only sister taxa. The tracks in Poland mean that the transition in some forms occurred earlier than we thought. It doesn’t mean that lobe-finned fish ceased to exist. Evolutionary theory didn’t fail. It found a transitional form where they thought a transitional form would be. It failed only if they were trying to find the “missing link.” BTW, there are no amphibian tracks in the Grand Canyon dated to 540 million years. If you can find a scholarly article that posits that, please sent it on.

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No, James, I am not limiting my thinking to a unilineal approach. I agree that according to the theory, transitional animals do not only exist after the parent species and before the daughter species, but can also exist long after the daughter species. But that reduces its predictive powers. If tetrapod footprints exist before the assumed transitional fossil, then we do not have evidence that it is transitional. All we have is speculation. It is for that reason you state that evolution theory cannot determine direct ancestors, only similar taxa. But I disagree that the tracks in Poland mean that the transition occurred earlier than thought. The tracks in Poland indicate nothing about transition, but only indicate that tetrapods existed earlier than thought. We must realize that transitional forms are only assumed to be transitional. We have no proof that they are not dead-ends,(endpoints) for example.

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Yes you are still thinking unilineally only. You forget about collateral ancestry. Just because you have tetrapod tracks earlier than Tiktaalik does not make Titaalik any less transitional, it simply means that it was a one of a whole bunch of transitional fossils that were running around at the time, some later than others. Evolution proceeds in mosaic fashion and from the first critter with transitional traits to the last one can be a long time. Look at birds. The earliest traits start showing up in the Triassic, but true birds don’t show up until the Cretaceous.

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Sure, I understand what you are saying. But nevertheless, it is speculation. You don’t know if its transitional. It looks like it might be, but you don’t know if it is, or if it is an endpoint, and didn’t transition to anything at all. The tracks say absolutely nothing about the ability of Tiktaalik to be transitional, other than that it does not have to be transitional, because previous tetrapod footprints indicate that something else was transitional before tiktaalik. Or of course, maybe tiktaalik existed at an earlier time. But then maybe the tetrapod also existed at an earlier time as well. Just because we haven’t seen either the tetrapod footprints or a tiktaalik fossil, doesn’t mean they were not there. Obviously.

Whether evolution proceeds in a mosaic fashion is a theory which seems nebulous in terms of lack of specificity. It seems to say that there were a whole bunch of transitional animals and species that led to a whole bunch of other species. You seem to be arguing that there wasn’t one fish that began to walk, but a whole bunch of different fish of various sizes that developed all kinds of feet and different patterns of toes, turning into a wide variety of land animals such as amphibians and reptiles and seals and mammals. You are proposing a sort of lack of progression, or you are increasing the improbability of the whole thing because of the multiplicity of positive genetic events that would be required. You might be forgetting that in this great mosaic of yours, most of the apparent transitional fossils probably represent dead ends (transitioning to nothing), and we would still have to look for the real transitionals which not only survived but actually mutated beneficially to produce the new limbs and organs. How would you distinguish between a dead end and a real transitional in your mosaic model?