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Debate Flares over Identity of Celebrated Human Fossils

The views expressed are those of the author and are not necessarily those of Scientific American.


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Australopithecus sediba lower jaws

Lower jaws, or mandibles, attributed to early human ancestor Australopithecus sediba differ in the shape of the so-called mandibular notch--the indentation in the top portion of the bone. Two paleoanthropologists contend that the difference signals that the top mandible actually comes from a member of our own genus, Homo, whereas the bottom one comes from Australopithecus. Image: Kate Wong; casts of A. sediba mandibles courtesy of Darryl de Ruiter

CALGARY—In 2010 paleoanthropologists announced to great fanfare that they had recovered from a South African cave two partial skeletons of a previously unknown member of the human family that lived nearly two million years ago. The skeletons—a young male and an adult female referred to as MH1 and MH2, respectively–were said to exhibit a striking melange of traits associated with our own genus, Homo, and traits characteristic of the older, more apelike genus Australopithecus, from which Homo presumably evolved. Its discoverers thus proposed that the creature, which they named Australopithecus sediba, could be a transitional species between the two groups, and thus the long-sought ancestor of Homo or a close relative of that ancestor. Critics countered that the fossils were too young to be ancestral to Homo, among other objections to that evolutionary scenario. Now two researchers have put forth an entirely different interpretation of the remains.

Addressing the Paleoanthropology Society in Calgary last week, Yoel Rak and Ella Been of Tel Aviv University made the case that the bones do not represent a transitional species but instead come from two different genera. In his presentation, Rak focused on the form of the lower jaw, or mandible. Specifically he looked at the dimensions of the so-called mandibular notch–an indentation in the uppermost portion of the bone. According to Rak, this aspect of the mandible differs reliably between Australopithecus and Homo, and the two lower jaws of the A. sediba individuals differ along those lines. MH1 has a mandibular notch like that of Australopithecus; MH2 has one like Homo’s.

Been, for her part, assessed the four lumbar vertebrae of A. sediba and arrived at a similar conclusion. For each of the specimens she considered the front-to-back length of the central portion of the bone (the vertebral body) against the height of the entire vertebra. Comparing the resulting ratios to those obtained for lumbar vertebrae from modern Homo, early Homo and Australopithecus, she determined that the MH1 lumbar vertebrae align with Homo yet the MH2 lumbars group with Australopithecus.

That MH1 seems to have an Australopithecus-like mandibular notch and Homo-like lumbars, whereas MH2 combines a Homo-like mandibular notch with Australopithecus-like lumbars indicates to Rak and Been that each alleged skeleton is actually a mix of bones from Homo and Australopithecus, for a total of four individuals in all.

Australopithecus sediba skeletons MH1 (left) and MH2 (right) flank the famous Australopithecus afarensis skeleton known as Lucy (center). Image: Compiled by Peter Schmid courtesy of Lee R. Berger, University of the Witwatersrand/Via Wikimedia Commons

That interpretation does not sit well with members of the Malapa team. Darryl de Ruiter of Texas A&M University acknowledges that the shape of the mandibular notch differs in MH1 and MH2. He disputes the idea that this single feature can be used to diagnose genera, however. “The pattern is real,” he says, “but the meaning [Rak] attaches to it is where I disagree.” Likewise Marc Meyer of Chaffey College notes that he and others tasked with evaluating the A. sediba vertebrae take 70 measurements per bone and generate dozens of indices. “I can cherry-pick one index and turn sediba into a chimp; another turns it into a human,” he comments. “Generating a taxon-level conclusion from one index is pre-mature.”

One of the challenges paleontologists face in dealing with the imperfect fossil record is that of distinguishing variation within a species from variation between species. Complicating matters in the case of A. sediba, MH1 is a juvenile and MH2 is an adult, which means that some of the differences between them may simply reflect the incomplete growth and development of MH1.

But most problematic for the argument that MH1 and MH2 comprise two genera and four individuals is the fact that the specimens were found as articulated skeletons, with the bones in anatomical alignment, and without any duplication of parts to indicate that more than two individuals are represented. “There are not two genera,” states Lee Berger of the University of the Witwatersrand, who led the recovery of the fossils. He points out that MH1 and MH2 share a number of characteristics with each other that are not seen in any other member of the human family. “The time has come to stop looking for silver bullet traits for questions of higher taxonomy,” Berger asserts. “When you have fantastic partial skeletons you need to look across the entire skeleton.”

 

 

 

 

 

 

 

About the Author: Kate Wong is an editor and writer at Scientific American covering paleontology, archaeology and life sciences. Follow on Twitter @katewong.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. tuned 11:12 am 04/17/2014

    Nothing uncommon about archeology/ paleoanthropology fraud
    for fame and glory.
    Doesn’t change my day any. X>

    Link to this
  2. 2. kwong 11:23 am 04/17/2014

    No one is accusing anyone of fraud here. There are just different interpretations of what species these fossils come from.

    Link to this
  3. 3. greg_t_laden 12:30 pm 04/17/2014

    Australopithecus sediba is an important and amazing find, but I’m agnostic at the moment regarding its taxonomy. But, it might be pointed out that if sebida is an Australopith directly ancestral to Homo (representing, in other words, a species at or very near the last common ancestor of early Homo and other Australopiths) than we might expect certain features to appear Australopith like in the juvenile form and Homo like in the adult form.

    Link to this
  4. 4. SteveO 12:33 pm 04/17/2014

    The photo seems to indicate MH1 and 2 are the same size, so that would argue against age differences, but could it be gender differences?

    Link to this
  5. 5. greg_t_laden 1:32 pm 04/17/2014

    Actually they are very different in size, and the teeth seem to show very different age.

    Link to this
  6. 6. 1erasmus 1:46 pm 04/17/2014

    The mandible of MH1 is roughly similair, or at least large in comparison, to MH2, even though MH2 is obviously much older and MH1 wasn’t even fully grown (estimated about 95% of adult brain size). This has been interpreted as evidence of sexual dimorphism in the species.

    If Au. sediba is a transitional species, or at least closely related to “the missing link”, the fact that there has been so much argument over it’s classification as Homo or Australopithecus sort of stands it in good stead for that claim. Surely a transitional species should be difficult to definitively classify one way or another.

    Link to this
  7. 7. BookSpine 7:32 pm 04/17/2014

    kwong,

    Some people see wickedness in the most mundane of differences of opinion.

    Link to this
  8. 8. rossm 9:01 pm 04/17/2014

    Is there any research on which bone features are genetically determined, and which are caused by environmental changes. For example, could a significantly tougher diet, requiring a lot of molar chewing, change the shape of the jaw?

    Link to this
  9. 9. Bill_Crofut 9:01 am 04/18/2014

    The title may be indicative of a positive claim:

    “Neither the hypothesis of continental drift nor that of evolution was proved true before it won acceptance. In fact, no scientific hypothesis is ever proved true. Rather, it is tested continually with more and more data, and as the data continue to conform with the idea, and fail to disprove it, the community of scientists comes to have more and more confidence in its validity.”

    [Prof. Douglas J. Futuyma. 1982. Science on Trial: The Case for Evolution. New York: Pantheon Books, p. 67]

    On the other side, it may be recognized as less than positive:

    …[I]t does appear to me…that Darwin in the Origin was not able to produce palaeontological evidence sufficient to prove his views but that the evidence he did produce was adverse to them; and I may note that the position is not notably different to-day. The modern Darwinian palaeontologists are obliged, just like their predecessors and like Darwin, to water down the facts with subsidiary hypotheses which, however plausible, are in the nature of things unverifiable.

    [Prof. W. R. Thompson. 1956. Introduction. In: Charles Darwin. Origin of Species. Everyman Library No. 811. London: J. M. Dent and Sons. Reprinted with permission. Evolution Protest Movement. 1967. NEW CHALLENGING ‘INTRODUCTION' TO THE ORIGIN OF SPECIES. Selsey, Sussex: Selsey Press Ltd., p. 15]

    Link to this
  10. 10. Mendrys 4:15 pm 04/20/2014

    Bill,

    Why would you bring mention of a rewritten introduction of “Origin of Species” brought to us by people whose faith is somehow shaken by some inconvenient truth’s? People who can’t stand the idea that the Earth, heavens and sky was maybe, just maybe, not created just for them? Also, you may want to recheck but I think the Catholic church left you behind a few centuries ago.

    Link to this
  11. 11. DavidMarjanovic 5:36 pm 04/21/2014

    Bill_Crofut has been mentioning this stupid introduction from 1956 for years! Several people, myself (a paleontologist) included, have tried to explain to him several times that enormous amounts of fossils have been discovered in the last fifty-eight years, and that “unverifiable” is bad science theory in the first place; he never so much as takes notice.

    we might expect certain features to appear Australopith like in the juvenile form and Homo like in the adult form

    Or the other way around.

    Link to this
  12. 12. hkraznodar 6:04 pm 04/21/2014

    @Mendrys – You are correct. The Catholic Church as well as all of Islam left Bill far behind centuries ago.

    Link to this
  13. 13. Bill_Crofut 11:36 am 04/22/2014

    Mendrys,

    Re: “Why would you bring mention of a rewritten introduction of “Origin of Species”

    The document was not rewritten. It is a reproduction of the “INTRODUCTION” to one of the 1956 editions of Darwin’s novel in booklet form. Your gripe is not with those who obtained permission to reprint the intro, but with the publishers.

    The Church has not, nor will ever, leave me behind. Many individual churchmen have indeed left me behind. That does nothing to change consistent Church Teaching. It merely displays an intentional disregard for that Teaching by many members of the Church.

    Link to this
  14. 14. Bill_Crofut 12:04 pm 04/22/2014

    Dr. Marjanovic,

    Why do you consider the introduction to one of the 1956 editions of Darwin’s novel, stupid?

    Re: “Several people, myself (a paleontologist) included, have tried to explain to him several times that enormous amounts of fossils have been discovered in the last fifty-eight years…”

    You have indeed provided an impressive listing of fossils:

    Tyrannosauroidea
    Coeluridae/Compsognathidae
    Ornitholestes
    Ornithomimosauria
    Therizinosauria
    Yixianosaurus
    Oviraptorosauria
    Deinonychosauria
    Archaeopteryx
    Shenzhouraptor
    Jixiangornis
    Sapeornis
    Confuciusornis
    Enantiornithes
    Archaeorhynchus
    Patagopteryx
    Songlingornithidae
    Gansus
    Ichthyornis
    Hesperornithes
    Aves

    “Now let’s see which ones of these were known in 1956…”

    38. David Marjanovic 3:32 pm 08/12/2012

    Getting a major chapter on birds – ALL birds – into a major book on dinosaurs

    http://blogs.scientificamerican.com/tetrapod-zoology/2012/08/09/bird-chapter-in-complete-dinosaur-ii/?WT_mc_id=SA_DD_20120810

    So, the charge of never taking notice is false. You might wish to share with me the significance of the above-listed fossils.

    Link to this

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