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Of heads and headlines: can a skull doom 14 human species?

A newly discovered 1.8 million-year-old skull from Eastern Europe has been pitched as disproving a decades-old paradigm in human evolution. Its discoverers claim the find sinks more than a dozen species…

Spotting the difference between skulls - this is the Dmanisi D4500 early Homo cranium - is trickier than it seems. Photo courtesy of Georgian National Museum

A newly discovered 1.8 million-year-old skull from Eastern Europe has been pitched as disproving a decades-old paradigm in human evolution.

Its discoverers claim the find sinks more than a dozen species into a single evolutionary line leading to living people. But the new study highlights the propensity of some anthropologists to overstep the mark, interpreting the importance of their finds in a way that grabs the headlines.

More big claims

The more-than-150-year history of human evolutionary science is filled with many remarkable and headline-grabbing episodes.

Some of them were proved correct: Eugene Dubois’ 1891-92 discovery of Pithecanthropus (now Homo erectus), Raymond Dart’s 1925 announcement of Australopithecus africanus, and more recently, Michael Morwood and co-worker’s 2004 announcement of Homo floresiensis.

But today’s article in Science by David Lordkipanidze and co-workers is going to make an even bigger splash, by challenging a well-established paradigm.

They described and compared a new skull from the Dmanisi site in Georgia, dated to around 1.8 million years old. It is one of five skulls in varying states of completeness. The latest one (“Skull 5”) is well preserved and includes a matching lower jaw.

Until now, it was generally accepted that some of the Dmanisi skulls probably came from different species.

The five Dmanisi skulls. M. Ponce de León and Ch. Zollikofer, University of Zurich, Switzerland

However, Lordkipanidze has said previously that all of the human remains from Dmanisi are a single group, perhaps killed in a natural disaster. This is possible but difficult to substantiate, because of errors in dating methods.

Using three-dimensional computing, this new study now also claims the differences in “shape” among the five ancient Dmanisi skulls is no more pronounced than observed between five living humans or five chimpanzees.

This is despite the fact that when the anatomical features (such as the eyebrow bone) of the skulls have been examined by experienced biologists rather than abstract computer methods, researchers like Jeffrey Schwartz have suggested that the Dmanisi sample contains multiple species.

Big blow to diversity?

But here’s the potential killer blow for anthropology. Lordkipanidze and colleagues say their work shows the entire early fossil record for Homo – comprising perhaps nine species dating between about 2.3 and 0.5 million years old – is in fact a single long-lived group.

Here’s the list of now apparently defunct species: Homo habilis, Homo rudolfensis, Homo gautengensis, Homo ergaster, Homo georgicus, Homo soloensis, Homo pekinensis and Homo mauritanicus.

All of them would now be sunk into Homo erectus according to the study findings.

Their work also has major implications for later Homo, implying that a further six species should all be sunk into our kind, Homo sapiens. The species no longer required would be: Homo heidelbergensis, Homo rhodesiensis, Homo antecessor, Homo neanderthalensis, Homo helmei and Homo floresiensis.

So, the entire ~2.4 million years of evolution of the genus Homo comprises, this new study proposes, at most two species: Homo erectus and Homo sapiens. These would both belong to a single evolutionary line rather than being separate twigs within a bush of species.

Most anthropologists would currently recognise at least nine and up to 17 species of Homo, so the pruning would be about as radical as one can imagine!

History repeats itself

These latest suggestions about the lack of diversity within our evolutionary group hark back to the mid-20th century ideas of the evolutionary biologists Theodosius Dobzhansky and Ernst Mayr.

Homo habilis: no longer needed. Cicero Moraes

Dobzhansky wrote in 1944:

there is no reason to suppose that more than a single hominid species has existed on any time level in the Pleistocene.

Well, Dobzhansky and Mayr got it wrong. Mayr admitted this later in his career in response to the many dramatic fossil discoveries made from the mid-1960s onwards, especially in Africa.

The Dobzhansky/Mayr scenario is known as the “single lineage hypothesis” and the new research by Lordkipanidze and colleagues – a new version of it – implies that even this long-disproved idea exaggerated diversity in our tree.

End of a paradigm?

There are a number of things that stand out to me as worrying about the new Dmanisi research.

I don’t doubt the broader significance of the fossils or their worthiness of Science treatment. But I find the claim of a single evolutionary line, comprising one or two species of Homo, to be wildly premature.

This headline-grabbing approach to publication has become one of the pitfalls of modern academia. Let me explain.

Articles in journals like Science and Nature are highly sought after by universities around the world. This is in part because of the current obsession with international league tables. Some use the number of Science and Nature papers published as a criterion for judging quality.

The previous commonwealth government began explicitly linking university funding to publication quality, developing rankings of journals under its Excellence Research for Australia model.

If our senior peers deem our work good enough to be published in Science or Nature, we join the ranks of a select few. Our academic careers can benefit greatly from the high exposure these journals enjoy.

So there are reasons why a researcher would aim for the kind of headline-grabbing study Science would publish. But the scenario of human fossil diversity is now well founded and receives widespread support from the fossil record as well as broader evolutionary theory: throwing it out at this point would seem a tad hasty.

Revealing also is the article’s acknowledgement of a major contradiction between their 3D virtual skull shape analyses - suggesting very low species diversity - and their studies of anatomical features - which indicate very high species diversity in Homo.

In my experience, 3D approaches often fail to detect the subtleties of species differences, especially among genetically closely related groups like the species of Homo.

3D studies of skulls can be like comparing apples and plums: both are fruit, and spherical, but they have very distinct evolutionary origins and histories. If the quality you’re interested in is sphericity, then fine, but otherwise it’s a very blunt tool.

In contrast, anatomical characters are the mainstay in the description of species as well as for establishing evolutionary relationships, and are used right across the biological sciences.

To favour 3D results over anatomy seems to me to be highly selective and unjustified. In my experience, we should use both. Had they done so, the latest Dmanisi story might have been very different.

A question of inheritance

The shape and size of skulls is greatly affected by the environment as they grow during childhood, and although many genes are involved in the process, their influence seems to generally be quite small.

The later in life growth stops, the more influence the environment exerts over its final size and shape. Skull form can be greatly altered by poor nutrition or disease or even heavy use of a structure (like the jaws) at crucial times in childhood.

Studies also show that the environment, rather than genetics, probably plays a large role in determining the final shape of the skull as revealed with 3D tools.

Possible appearance of D4500 in ¾ view. Art courtesy of J.H. Matternes]

In contrast, many anatomical features of the skull, especially features of the face like chins or eyebrow bones, are routinely used to diagnose species in the human fossil record. They tend to be subject to greater genetic and less environmental influence during growth.

A related problem here is “homoplasy”. This describes a situation where features are acquired by two species through separate evolutionary events, and not inherited from a shared (common) ancestor. These features are notoriously difficult to identify, can’t tell us about evolutionary history or relatedness, and are a source of error.

While such features can be strongly heritable or environmentally influenced, 3D analyses give equal weight to all traits, and so can be misleading. Homoplasy is not distinguished from genuine and evolutionarily informative traits.

Homoplasy is a well-known problem that confounds interpretations of the human fossil record. You ignore it at your peril.

Keeping the baby and the bath water

There are important lessons to be learnt here. By all means use 3D techniques, as they offer exciting new ways to explore and quantify biological form. I use them myself and they can potentially deepen our understanding of evolution when deployed in the right way.

But, to do so in a way that is divorced from basic biology is little more than an exercise in “black box” science.

The paradigm of diversity for human evolution will remain intact despite the new Dmanisi discovery. It’s unlikely to be thrown out just yet, let alone on account of a single skull or a trendy statistical technique.

Dmanisi is a very important site and its fossils crucial to properly understanding the course of human evolution. The variation seen among the five skulls without doubt has something to tell us about evolution. But it’s not the yarn being spun by the latest headline-grabbing Science article.

I think it’s a shame that the importance of the find, as with Australopithecus sediba, will now be overshadowed by protracted debate about exaggerated interpretations and hyperbole.

While such controversies are not unique to anthropology – think of the ENCODE junk DNA debate earlier this year – the discipline does have a rather long history of them.

Join the conversation

20 Comments sorted by

  1. Gavin Moodie
    Gavin Moodie is a Friend of The Conversation.

    Adjunct professor at RMIT University

    Thanx for this most interesting report and analysis.

    While primary responsibility for exaggerated or premature claims clearly lies with the authors, surely some responsibility must be borne by the journals: their editors and referees. Science and Nature seem to put commercial considerations before accuracy in some of these spectacular claims.

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  2. Chris Booker

    Research scientist

    Great write-up, I particularly like how you've called out the "Science" factor of over-hyping research results.

    I think some of this can also be traced back to the same old Linnaean dilemma of where you draw the arbitrary line between what is one species and another, which becomes especially important when examining the evolution of species thought to be in the same lineage; inevitably our definitions are just lines in the sand placed on a very long continuum.

    I have little understanding of the ins and outs of hominid evolution, but my reaction when I saw this in the news was that these researchers have just been attempting to take a very 'macro' level, all-encompassing view of what a species is or is not, and thus end up with two species where previously there were many; but it's still just an arbitrary cut-off.

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  3. David Arthur

    resistance gnome

    Thanks for this article.

    Perhaps publication in "Science" was intended to engender a large debate; A/Prof Curnoe points out, however, that such debate is around whether or not species differentiation is exclusively determined by skull structure. At least it's a relatively testable, and hence falsifiable, hypothesis.

    My understanding is that numerous detailed studies of homo floresiensis have already shown sufficient differences for it to be distinct from homo sapiens - so how did all those studies evade the reviewers of the draft paper for them to accept its classification of h. floresiensis with h. sapiens?

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  4. Mike Brisco

    Scientist at Flinders University of South Australia

    Couple of things.

    One, where's the repetition? It is dangerous to erect a theory on a sample size of one e.g one skull.

    Two, Popper cautioned us, that to reject a theory, the weight of counter-evidence varies. A speculative theory that hasn't been tested much - a single counter-example can fell it. But a theory that stood up to testing for years - requires more counter-evidence before it's declared falsified.

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  5. Brandon Young

    Retired

    Defending the paradigm against the black boxes may not ultimately serve the collective goal of the development of knowledge, although the psychology involved is fairly obvious.

    A better system would ensure the black boxes were valued whatever the degree of uncertainty, and that credibility was awarded for every individual contribution to the expansion of objective knowledge.

    But change will be slow, and serious consideration resisted, when ego and status are threatened.

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    1. Matt Stevens

      Senior Research Fellow/Statistician/PhD

      In reply to Brandon Young

      Totally agree, let's push another wheel barrow shall we?

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  6. John Crest

    logged in via email @live.com.au

    "Dobzhansky wrote in 1944: there is no reason to suppose that more than a single hominid species has existed on any time level in the Pleistocene."

    Shouldn't "Well, Dobzhansky and Mayr got it wrong" be more accurately expressed as "Dobzhansky's views are not in accord with the currently accepted scienjtific paradigm"?

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  7. Anvil Springstien

    scribbler and talker of sorts at 'Near the Knuckle Productions'

    A thought provoking and stimulating article.

    "While such controversies are not unique to anthropology (...) the discipline does have a rather long history of them."

    Yes, but surely this is the nature of the discipline itself. It is hardly - I'm going to hate myself for saying this - 'Rocket Science' is it?

    A bone fragment here, a jaw-bone there, a fossilised skull deformed by deep time? On their own and without a nod to other fields such as biology, molecular biology, et al, there can be…

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    1. Chris Booker

      Research scientist

      In reply to Anvil Springstien

      "On their own and without a nod to other fields such as biology, molecular biology, et al, there can be little beyond strong hypothesis, can there"

      I'd really like to see this field move into a state where findings like this are published with genome sequencing as a core component of the paper. Even if there's no recoverable DNA, to at least try it should be regarded as one of the key undertakings needed for publication. Doing these kind of skull measurements, whether by 3D tools or otherwise, without any regard to molecular biology seems like an awfully outdated approach to examining hominoid evolution.

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    2. Anvil Springstien

      scribbler and talker of sorts at 'Near the Knuckle Productions'

      In reply to Chris Booker

      Yes, I agree, Chris. As I said, it's not my field so my intention was not to imply that this is the present state of play in anthropology. I simply don't know if this is the case.

      That said, being unable to answer the question 'What did the molecular analysis of the find yield?' when presenting your paradigm shattering research to the world would seem odd at best - and at worst neglectful?

      I'd presumed things had moved on?

      I'd have thought by now that all anthropologist would be armed with a Star Trek like implement or iPhone tunnelling scanner app' thing that beeps and say's "It's a Hominid, Jim, but not as we know it?"

      One day, eh?

      Anvil.

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    3. Matt Stevens

      Senior Research Fellow/Statistician/PhD

      In reply to Chris Booker

      They can't extract dna, it is all gone. But at least using 'black boxes' as the biassed author points out provides a more objective measure than a subjective one that supports what they have believed for however long.

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    4. Chris Booker

      Research scientist

      In reply to Matt Stevens

      Yeah I guess I was meaning more in the coming years - the age of readable DNA is constantly being pushed back and in all honesty I think it's only a matter of time before sequencing samples like these becomes possible. Also, when you take a look at that skull it's in good condition, and every cell is loaded with a whole genome's worth of DNA, so there's still plenty there, it's just in fragments which may not be readable with current sequencing methods. We already have a Neanderthal genome, for example. In addition, it would interesting to see whether some of the newer direct sequencing technologies (i.e. without the need for amplification) may yield better results on these kinds of samples than current methods.

      Personally, I think when assembling genomes from samples like this becomes possible, then we will really see some results which challenge the currently accepted lineages.

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    5. Anvil Springstien

      scribbler and talker of sorts at 'Near the Knuckle Productions'

      In reply to Matt Stevens

      "They can't extract dna, it is all gone."

      Well, yes and no. The existence, or otherwise, of amplifiable DNA is dependent on many factors other than just mineralisation. Not least of these is the treatment of the find at the point of excavation.

      Anvil.

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  8. Murray Goulden

    Research Fellow at University of Nottingham

    Another fascinating dispute. I had the good fortune to study the science of paleoanthropolgy from a sociological perspective for my thesis, and it made for fantastic material! The combination of culturally-loaded subject and fleeting evidence is a volatile cocktail.

    Apologies for the self-promotion, but those interest by Darren's articles might also be interested in my work on how science and the media dealt with Floresiensis and Piltdown man.

    http://pus.sagepub.com/content/22/5/575.abstract
    http://pus.sagepub.com/content/18/3/275.short
    http://www.tandfonline.com/doi/abs/10.1080/09505430701706699#.UmEFnvIQNRU

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    1. Matt Stevens

      Senior Research Fellow/Statistician/PhD

      In reply to Murray Goulden

      Yes, a very loaded field of research indeed...thanks for the links.

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    2. Anvil Springstien

      scribbler and talker of sorts at 'Near the Knuckle Productions'

      In reply to Murray Goulden

      Nice, Murray, that must have been fun!

      If the abstract is anything to go by it should be prescribed reading for all first year PA's

      A front page opportunity to get those Darwinian synapses sparking about an issue no less important than the origin of our species and our standard teleological view of it.

      Anvil.

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  9. Matt Stevens

    Senior Research Fellow/Statistician/PhD

    Thanks for the article but my biassed view would lead me to trust the more objective 'trendy statistical technique' as opposed to the subjective biases of an anthropologist with a reason to go in to bat for their theory.

    funny, having studied evolutionary ecology I though the definetion of a species involved not interbreeding or an inability to. Yes this species separation can be by geograohy, but evidence suggests that neadertal, denisovian and homo sapien all interbreed. Not really a defintion of a species, particularly one as mobile as what home sapien aka human are.

    Yes, I trust the objective 'black box' method over the subjecyive anthropologist any day. :-)

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    1. Don Gibbons

      Clerk

      In reply to Matt Stevens

      Hi Matt

      Using the capacity to interbreed as a species definer, is, as you've said, one of several methods that have or are being used to define species. It has little utility for palaeontologists, who perforce have relied on morphological methods. Defining species in palaeontology is further complicated by change and diversification through time and space which is frequently masked by discontinuities in the fossil record and small sample sizes (not just the number of individuals, but the parts of those individuals recovered). It seems to me that whether you are a lumper or a splitter of species, however you define species, that judgement necessarily contains many arbitrary judgements. Even cladistic analysis of phylogeny, which tries to maximise objective assessment, includes subjective judgements as an essential element; http://www.ucmp.berkeley.edu/clad/clad2.html

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  10. Carl Sholin

    logged in via LinkedIn

    I welcome the new hypothesis. Gavin Moodle, I'm not exactly sure that the responsibility does lie primarily with the authors. The enterprise of science is built upon making bold claims and then leaving it up to the community of researchers to either support or refute these claims. With specific regard to Hominin evolution the prevailing paradigm has been cobbled together from several fossil discoveries made over the last century. Typically when a new discovery is made there is pressure on the researcher to classify it within the structure of this prevailing theory. I agree with Chris Booker, that usually these distinctions are based on arbitrary definitions of how a species is defined. I don't think that our understanding of Hominin evolution will really progress unless researchers put forward alternative models.

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