A new and important hominin skull from Ethiopia

September 1, 2019 • 9:45 am

A new analysis of a remarkable hominin find in Ethiopia suggests that the species it represents, Australopithecus anamensis, may be one of the very earliest species in our lineage, and possibly the first hominin we know of that is undoubtedly part of our own genealogy. (“Hominins”, formerly called “hominids”, represent all fossils on our side of the family tree since we branched off from our common ancestor with the chimpanzees). The find has also has led to a revision of the idea that A. anamensis was the ancestor of the later A. afarensis, thought by many to be the ancestor of the genus Homo, and thus of modern Homo sapiens. (We love to know who our ancestors are, as witnessed by the popularity of companies like 23 & Me.)

A. anamensis lived from about 4.2 to 3.9 million years ago (mya) and A. afarensis from 3.9 to 3.0 mya. A. afarensis includes the famous skeleton of Lucy(about 3.2 mya), which is remarkably complete from the neck down but has only fragments of the skull.  As the following diagram suggests (and this is typical), A. anamensis is the earliest known hominin to be part of our own lineage and is portrayed as the ancestor of A. afarensis:

That ancestry is presumed (as shown above) to have been a lineal transformation of A. anamensis into A. afarensis: that is, the former species was thought to have evolved over time, and as a unit, into A. afarensis.

This conclusion is what is cast into doubt by the recent find, documented in a new paper in Nature that you can access by clicking on the screenshot below (or see the pdf here). The new finds show that A. anamensis was in fact a contemporary of A. afarensis, so that the two species lived at the same time, at least for a while.

This doesn’t rule out, however, the possibility that A. afarensis evolved from one or more populations of A. anamensis while the latter species continued on, largely unchanged from other populations. (Some, however, would say that such a branching event would automatically change the name of A. anamensis; see below.) Thus both species could still have coexisted while A. afarensis could still be a descendant of A. anemensis.

The paper:

As you see, the new skull is 3.8 million years old, putting it at about the end of A. anamensis‘s tenure and about the time that A. afarensis appeared.

The new skull is remarkably complete: by far the most intact A. anamensis skull we have, and also has a number of features that tell us that it was a hominin and was not a member of A. afarensis. Since the earliest A. afarensis appears to have lived a bit earlier than this specimen (3.9 mya), it appears that A. afarensis cannot be a lineal descendant of A. anamensis. (But, as I said above, it could be a descendant of some A. anamensis populations.)

Here’s the skull, which has a tiny cranial capacity (365-370 cc compared to about 1200-1300 cc in modern humans), a sagittal crest, and brow ridges.

a, Anterior view. b, Posterior view. c, Superior view. d, Left lateral view. e, Right lateral view. f, Inferior view. The specimen is oriented in Frankfort horizontal plane. Scale bar, 1 cm.


You can see how small the skull is when compared to the head size of the paper’s first author:

Yohannes Haile-Selassie with the skull. Photograph: AFP/Getty Images

It appears to have been an adult male.

Why is it a hominin? Well, here’s the jargon that they use to show it’s a hominin and more primitive (that is, closer in appearance to the common ancestor) than is A. afarensis:

The specimen is readily identifiable as a hominin by the following morphological features: the canine is reduced in size compared to non-human apes and shows a strong lingual basal tubercle; the mastoids are inflated; the nuchal plane is more horizontal than in non-human apes; and the inion, which is coincident with the opisthocranion, lies near the level of the Frankfort horizontal plane. At the same time, the small cranial capacity, highly prognathic face, extensive pneumatization and other features discussed below indicate that MRD represents a hominin that is more primitive than A. afarensis.

Got that? Neither did I, but the paleoanthropologists do.  At any rate, the dates put it earlier than most specimens of A. afarensis, and it has a longer upper canine, a smaller earhole, and a narrower palate. Here’s a comparison of the new specimen (MRD-VP-s/1, which we’ll call MRD) with Sahelanthropus tchadensis (a hominin that may be close to the common ancestor of the human and chimp lineages, and which lived about 7 mya), along with Australopithecus ramidus (a 4.4 mya hominin of unknown placement on the tree), and A. afarensis and the later A. africanus.

(All captions below are from the Nature paper): Red lines and arrows show the inclination of the frontal and the presence of a post-toral sulcus, respectively. Blue lines show the orientation of the mid and lower face, with an broken line indicating a segmented facial profile27. The green arrow marks the anterior projection of the zygomatic tubercle (relative to the anterior zygomatic root). Scale bar, 2 cm.

Here’s the back of the skull compared to a modern chimp and the two later australopithecines. Note the smaller braincase and more pronounced sagittal crest of MRD:

The transverse contour of the cranial base is convex in African apes, whereas A. afarensis shows an angular transition between the nuchal region and the greatly expanded mastoids (red dashed lines). In this regard, A. afarensis anticipates the morphology of robust australopiths, but A. africanus is less derived. MRD shows the primitive convex contour of the base, even though the mastoids are expanded. MRD is also primitive with regard to the great length of the nuchal plane (black arrows). However, it is similar to A. afarensis in the configuration of the compound temporal–nuchal crest (white dashed lines), the bare area (blue hatched triangle), and the overall ‘bell-shaped’ posterior outline (that is, the parietal walls are slightly convergent superiorly and the greatest width occurs basally across the enlarged mastoids).

A phylogenetic analysis of where the MRD skull fits on the hominin phylogeny. As you see, it appears before A. afarensis and sits on part of the phylogeny that gave rise to modern humans.

h, i, Cladograms from the K-combined and S&G-combined analyses (as in a and b), with apomorphies added to the cladograms to illustrate the implied pattern of evolutionary change. The character states reconstructed at nodes A and B provide the reference for identifying A. anamensis and A. afarensis apomorphies, which are shown here as rectangles containing their abbreviated character labels. Characters in red, orange, gold and green describe similar morphology and appear in both previously published studies27,33. See Supplementary Note 9 and Supplementary Table 1.

This species was sexually dimorphic, with males about five feet tall and weighing about 100 pounds (1.5 meters and 45 kilos), and females about 3.5 feet tall and weighing about 62 pounds (1.1 meter and 28 kilos). The skull size and features (tooth wear, etc.) suggest that it was an adult male. Here’s a reconstruction of the face from The Guardian:

The big findings: First, we now know what the cranium of a possible ancestor looked like, and crania are not easily preserved or found in the fossil record of hominins. This helps complete the picture of what A. amanensis looked like.  Further, another major and unambiguous conclusion here—assuming the dating is correct—is that A. anamensis coexisted with A. afarensis for at least 100,000 years.  A. afarensis is thought to be one of our ancestors, giving rise to the genus Homo as well as the extinct “robust” australopithecines. But this leaves one question:

Was A. anamensis an ancestor of A. afarensis? Just because the two species coexisted does not mean that the one who lived first wasn’t the ancestor of the one that appeared later. Consider that there were various populations of the earlier A. anamensis. Suppose one or more of these evolved into A. afarensis, but some other populations retained the appearance and traits of A. anamensis. Then we’d still have an ancestor/descendant relationship, though cladistic taxonomists would say that at the moment A. afarensis branched off, we’d have to change the name of A. amanensis. (This is part of the practice of cladistic classification, though it makes little sense to laypeople.)

Whether A. afarensis came into being this way or not, what’s clear is that two species of australopithecine lived at the same time, and one of them is thought by paleoanthrophologists, as the first diagram above shows, to have been the ancestor of several species of hominins, including us.

h/t: Pyers

55 thoughts on “A new and important hominin skull from Ethiopia

  1. This is rather common, isn’t it? I mean an ancestor species overlapping with a decedent. Is there any indication that afarensis and amanensis lived in the same places?

    1. Well, with polyploid plants it happens all the time: a new autopolyploid, for example, could form by doubling the chromosomes of an existing species, and then find an ecological nice. (Allopolyploids form after hybridization, but can also live near their two ancestral species.) After the polyploidy formation, we have the ancestor still there, but new descendant species.

      I can’t think at the moment of non-polyploid species in which this is the case, but it certainly must have happened–surely with ancestral species that invade islands and stay pretty much the same on the mainland while the island invader becomes a new species. So no, this isn’t an aberrant thing to happen in evolution, but the authors of the paper don’t seem to accept that this could have happened with these two species.

      1. Are there any Homo Erectus specimens thought to have come from later than some H.Floresiensis (“hobbit”) existed? Related: is H.Erectus thought to be a ‘single species’ in some strong sense of that phrase?

      2. Thank you. I was, in fact, thinking of Darwin’s finches. Surely at least some of their ancestors were alive when they diverged on the islands. Perhaps afarensis was reproductively isolated by geography.

  2. I thought that the more common pattern of evolution of new species is that small, reproductively isolated populations are the engines of evolution because, as a small population presumably lives in a relatively small area, the individuals are more likely to face consistent selection pressures across their range.

    Is this wrong?

    I first saw this news on the BBC Science page, misleadingly entitled “‘All bets now off’ on which ape was human ancestor”, but when I read the article my first impression was “Cool! Allopatric speciation!”

    1. Well, we’re not sure how small populations that are the ancestors of new species really are. In the cases of island invasion, they must be small, sometimes even a single pregnant female! But the relative importance of large versus small populations in speciation is something that is hard to address. New island endemics with relatives in other places surely arise this way, but we have lots of new species forming on continents and in the oceans as well.

      As for Gould and other peoples’ ideas that small populations are NECESSARY for speciation because speciation requires genetic drift and the crossing of adaptive valleys, well, that idea is pretty much dead in the water.

      1. “..even a single pregnant female!” Presumably with at least one male fetus! Pretty dodgy genetically, I suppose!

        1. Yes, of course, unless it’s an asexual organism. But I don’t see why it’s genetically dodgy. There’s no conceptual problem about founding a new population, which then becomes a new species, starting from one inseminated individual. And surely this must have happened.

          1. Sure, I think I understand, though not completely. I’m assuming no male accompanied the unfortunate female onto that virtually isolated island. I’m sure it hardly needs saying that ‘dodgy’ and ‘impossible’ are not identical in meaning.

            For a species where a male impregnating a female is necessary for the production of offspring, I’m likely quite naive about the probabilities of success into the far future for tiny isolated populations.

          2. The word “unfortunate” there was a reference to the plight of any individual swept away from ‘home’, perhaps on a log in a hurricane. But then she was fortunate to have survived at all.
            As far as founding a new species, she would be herself unaware of that. And independently of the improbable survival, I’d be inclined, as I implied, to think the probability of her descendants existing far enough into the future (or ‘going forward’ for anyone who shuns the word ‘future’!!) to become a new species is extraordinarily small, if a sexually only reproducing species to begin with–though not zero if there was a male fetus inside her.

            Sorry to belabour what had seemed obvious to me.

  3. “…greatly expanded mastoids …(of)… A. afarensis …”

    IIRC that bone in the skull behind the ear is especially solid, and the best place for the ‘ancient DNA workers’ to hope for desired fragments. Though way older than what they have yet done among our ancestors (but not some mammal species, again if my memory is correct), I wonder whether there is any chance at all of them reconstructing the genome of some Australopithecines?–or even Homo Erectus?

    1. “in the skull behind the ear”..yes, but a lot of viable DNA has come from the interior of any existing teeth.

      1. Actually it’s the petrous part, not the mastoid part, of the temporal bone, so I got that wrong–learn something every day I hope!

        However the abstract to
        says “The only skeletal element for which a systematically higher endogenous DNA content compared to other skeletal elements has been shown is the petrous part of the temporal bone.” I’d interpreted that to mean also teeth were much inferior, but maybe not or maybe they are wrong.
        David Reich’s book “Who we are and how we got here” on p. 94 (see also 201) says the petrous bone preserves up to one hundred times more DNA than most other skeletal parts for each milligram of bone powder.

  4. This looks like a good time to ask a question I’ve always wanted to ask: Why “hominin”? Maybe “hominid” was incorrect nomenclature all along. If so, how did “hominid” get traction?

    1. “Hominid” is the adjective and noun derived from Hominidae, the formal name of the family that includes modern man. Formerly, Hominidae included, roughly, the genus Homo and the australopithecines (one to a few genera, depending on your preferences). The family was then expanded to include at least some parts (e.g., Pan, the chimpanzees) of the family Pongidae. This meant that the term “hominid” included chimpanzees, but most people wanted a term that could be used to name the lineage that led to modern Homo after it split with the lineage that led to modern Pan. Taxonomically, the next rank below family is subfamily, but the subfamily Homininae, the hominines, is usually considered to include Pan as well. The next lower rank is tribe, in this case the tribe Hominini, or hominins. Under the expansive view of the family Hominidae (and subfamily Homininae), Hominini has the same content as the former Hominidae, which is why nowadays you usuall see the term “hominin”, but in older literature the equivalent term is “hominids”.

      (Note that names at particular taxonomic ranks in zoology follow certain rules. In the above, we can see that families end in “-idae”, subfamilies end in “-inae”, and tribes end in “-ini”, where “-” represents the root word, in this case “Homin-“.)

      [Update: I wrote this explainer while Jerry was replying with an explanatory link, and I did not see his reply until I posted mine.]


      1. This leads to a subsidiary question – do “order”, “family”, “subfamily” etc have definitions in the way that “species” does (fertile interbreeding etc)?

        E.g. why not have the two new levels between Order and Family (instead of between Family and Genus) so that Hominidae can stay the same as before?

        What’s the reasoning?

        1. Except for species, ranks are conventional. Willi Hennig introduced a convention of having all non-species ranks correspond to age of origin (older origin = higher rank), but this has not caught on; it does have the advantage of being an objective convention. The mammalogist Richard van Gelder argued that any two species that could hybridize and produce offspring (even if the offspring were sterile) should be in the same genus. I am attracted to this conception myself, especially as applied to mammals, but in birds hybridization between wildly different species are known, which would lead to perhaps unwieldy genera, and his suggestion, like Hennig’s, has not been widely adopted.

          The ranks used are based mostly on tradition– those used by Linnaeus, plus a few later additions. Some ranks are used for some groups of organisms, but not others. Though hominin is based on a tribe, tribes are infrequently used in vertebrate taxonomy in general. To get ranks in between, there are various greater than and less than prefixes– mega-, super-, sub-, parv-, infra-. ‘Suborder’ and ‘Superfamily’ would be between Order and Family, and yes, there would be a way to preserve the traditional meaning of Hominidae by using these ranks. There are way too many dichotomies in the phylogenetic tree for all of them to be given names, so there is some practical limit on the number of ranks used and taxa recognized.


          1. Great, thanks. Its a point that’s bugged me for a while, but I’ve never seen mentioned (in the sort of “popular” books/web pages that I’m likely to see).

            That definition of genus does indeed sound very appealing, shame it doesn’t quite work.

  5. Even as a biologist I don’t understand the idea that ancestral and descendant species can’t overlap in time. After all, my father was alive for about half of my lifetime.

    1. LOL. It’s part of the conventions of cladism, a taxonomic method that, by and large, I think is pretty good. But this bit I don’t really understand. It has to do, I think, with the “new” old species not comprising all the descendants of the ancestor that had the same name.

  6. Very interesting.

    The fact that we have only one species of hominin today makes it feel as if this should have been the case in the past. But the fossil record paints a picture of a thickly branching tree, with all the stems but one having died out.

    It is a fascinating study.

    1. Imagine what it could have been like – the racism would be astonishing. After all, we are all one species now and can demonstrate that by interbreeding; but if there were populations of very similar creatures that were “that different” … wow.

  7. It’s funny – to me – I was just reading about Lucy last night [*] and Lucy is Australopithecus afarensis. Then later in the book, it mentions Homo Australopithecus.

    I am puzzled if that is a typo. Because there was a graphic showing evolution proceeding from A. afarensis to Homo habilis all the way to Homo sapiens sapiens.

    [*] The book : A really short history of nearly everything by Bill Bryson

  8. I remember back in the 60’s following new fossil discoveries closely and being amazed by each new one as they came along. This is yet another piece of the puzzle. I have to think we’ll eventually have an excellent story of hominin evolution – though probably never a complete story. May the discoveries continue long after I’m gone.

  9. Very interesting. I did not know there were these various other features to ID a hominid (bipedal ‘ape’). I would go with a forward-ish position of the foramen magnum, which is the opening for the spinal cord under the skull.

  10. That facial reconstruction is amazing. I suspect it’s done with computer graphics.

    Thanks for the post PCC(E); as others have noted, findings like these enlighten as well as complicate.

  11. “That ancestry is presumed to have been a lineal transformation of A. anamensis into A. afarensis: that is, the former species was thought to have evolved over time, and as a unit, into A. farensis.”

    Does this mean that it was thought (and was/is possible) that a single species was isolated and evolved over time into a new species without interacting with any other species?

    1. No, just that mutations arose that transformed A. anamensis, as a whole, into A. afarensis. I didn’t meant that the transforming species didn’t INTERACT with any other species (I presume you mean physical interaction, not interbreeding).

  12. The skull is a “naturesend” of course.

    I have two quick contributions, its’ late here:

    – The fossil species dating overlap is commented on (and made much hay off by the authors of the two papers outlining the find and its date&context). This overlap has recently got modeled for testing how likely an overlap is by using fossil data, and a good introduction can be found here: https://arstechnica.com/science/2019/05/probability-helps-narrow-down-the-search-for-human-ancestors/ . The quick version seems to be that an overlap of 100 kyrs is not enough to rule out A. anamensis as an ancestor of A. afarensis in a branching process, consistent with what Jerry writes here.

    – The first trait that I know of that likely unarguably characterize fossil Homo with a significant – a very, very, very significant P = 3/10,000 for biology – difference has been found [ https://advances.sciencemag.org/content/5/8/eaax3250 ]. The low δ_44/42_Ca values in teeth enamel under sustained breast feeding gives such a slope fit difference (between 0-4 years, I assume). So people see that apes under dietary stress may breast feed young late, in orangutans up to 8 years of age, but those periods give limited enamel traces. Modern cultures differ in interest of course, but at least we know early humans were a) lucky since they survived close to extinction and b) fond of breasts, lol!

  13. The A. africanus specimen in the illustration is the famous ‘Mrs Ples’ specimen held in the Ditsong NH museum in Pretoria, and found by Robert Broom in 1947. When I was in the museum years ago I remember first seeing the secure room in which it was held (along with other specimens) which was labelled ‘The Broom Room’ and wondering, stupidly, why a broom cupboard needed such high security.

  14. As a practicing plant taxonomist, I am as appalled now by a bit of taxonomy used by cladists as I was when I first learned of it. It’s a consequence of the idea that speciation occurs by species A becoming divided and both parts of the species change, becoming descendant species B and C. Might happen. Often, though, what seems to happen is that species A goes along with no obvious changes, or no changes at all that go out of the standards we had for defining species A early in its existence. Meanwhile, Population X (usually but not necessarily a small population) changes, and changes, and becomes so different from A that we consider it a different species, B. Sometimes A and B can’t even cross, and are clearly different species.

    At no point did species A change. At no somewhat longer period of time did A change outside its expected normal variation. If species B didn’t exist, we wouldn’t even consider renaming later populations of species A. Therefore, one cannot justify naming later species A as species C, to contrast with species B.

    (I do understand the logic of those who advocate this. I just consider the argument specious, a matter of human words, having nothing to do with biology, which is what we’re supposed to be talking about.)

    Therefore, in my opinion*, Australopithecus anamensis and A. africanus could overlap in time AND A. anamensis could be the ancestor of A. africanus.

    * which is, of course, correct

  15. This is a fascinating addition to the story of hominid evolution. No doubt, like all such discoveries it will raise even more questions than it answers, but now we have an important missing piece of the puzzle. As Richard Dawkins said somewhere, we are lucky to have any fossils.

    I’m a bit puzzled, though, at the lack of reference to the discoverers. Who made the find? How long had they been working at the site? In cases like this, the lead discoverer(s) is/are usually widely feted, but this angle seems very low key. Was the leader Haile-Selassie, the man in the photo? (Intriguing that he has the same name as the former Emperor).

    Sometimes the story of an expedition is about as enthralling as the find itself,though no doubt that would take us too far afield in this context.

  16. Thanks for that, including the comments additional explanations, Prof Coyne. I’ve read various other accounts of this new(?) hominin skull but nothing so well covered. I thank you.

    1. I heartily agree with Reggie and with the many others here who have found this post very interesting and educational. Thank you, PCC!

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