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Scientists scrap over the place of “Ardi” in the trees–and in the primate family tree

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


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ardi human ancestor lineage evolution trees savannaA set of descriptions last year of a 4.4-million-year-old hominid dubbed Ardipithecus ramidus pegged the primate as a human ancestor that lived in ancient African woodlands.

Now some researchers aren’t so sure that "Ardi" lived in the forest—or even belongs exclusively in the human line.

Before Ardi, common evolutionary wisdom held that our ancestors began walking upright in large part due to a change in scenery. As African grasslands expanded and new residential opportunities arose, some primates began to explore a new, more efficient method of locomotion, leaving quadrupedal tree-dwelling to the monkeys.

Analysis of the soil, rocks, plants and animals around Ardi, however, appeared to Tim White, one of the October 2009 studies’ lead authors and a professor of integrative biology at the University of California, Berkeley, and his colleagues to indicate that she had primarily lived and died in a wooded area. Thus, if she had been an able upright walker (as they also proposed, though other researchers are not convinced it was her preferred way of getting around), the finding meant that our ancestors might have started to walk on two legs in an entirely different setting, for entirely different reasons.

Another group of researchers, however, has proposed that the evidence from Ardi’s environment indicates a "tree or bush savanna" that would have had less than a quarter canopy coverage, in a technical comment submitted to Science in November and published online May 27. Thure Cerling, a geochemist at the University of Utah, and his colleagues assessed carbon and oxygen isotopes in tooth enamel, soil samples, rock, aridity levels and other evidence from sites in different regions to conclude that Ardi most likely lived in an environment that was rather grassy and dotted with occasional trees as well as denser vegetation around rivers and streams. Cerling and his coauthors do not say whether they support the idea that early humans started walking in more open savanna areas, but they did conclude that White et al.’s " rejection of the savanna hypothesis is incorrect," Cerling said in a prepared statement.

Such thinking that Ardi and her kind would have "equally exploited all of the habitats available to it" would be "ecologically naïve," White and three colleagues contested in their response, also published online May 27 in Science. They argued that results from enamel analysis "should be used with caution," there was "no local evidence" for a large river channel to produce any riparian vegetation coverage and that the group’s definition of Ardi’s environment was too broad and should be focused on the immediate surroundings in which the fossils were found.

But what if this primate—forest-dweller or not—was not even on the human limb of the tree of life? White and his colleagues argued in several of the group’s 11 papers from October 2009 that Ardi lived after the human-chimp split. "Sufficient support for this claim, however, is lacking," Esteban Sarmiento, of the Human Evolution Foundation, wrote in another technical comment in the same issue of Science. Based on descriptions of some of the teeth, skull, hip bones and foot bones, he argued that rather than suggesting that Ardi’s rightful place is in the human lineage, the data "suggest that Ardipithecus belongs to a common human/African ape lineage." And because the species is thought to have lived relatively close to the divergence of human and African apes, such as chimpanzees and gorillas, Sarmiento continued, "it would be difficult to unambiguiously recognize it" as a human-only ancestor.

In a rejoinder from White et al., they noted that in order for many of the characteristics described in Ardi to be ancestral to African apes as well, "a complicated and non-parsimonious sequence of…transformations would be required." Additionally, they noted that although Sarmiento disagreed with their interpretation of many anatomical characteristics, he didn’t offer satisfactory alternative explanations of his own. "If Sarmiento wishes to…generate phylogenic conclusions different from ours," White and his colleagues wrote, "then he needs to be explicit as to where, why and how our assessments of these characteristics were in error."

Regardless of where Ardi belongs in the primate tree—or whether she lived in the trees, Sarmiento noted that the description of this species follows a well-worn path in paleoanthropology: "It is curious that in a century-old race for superlative hominid fossils on a continent currently populated with African apes, we consistently unearth nearly complete hominid ancestors and have yet to recognize even a small fragment of a bona fide chimpanzee or gorilla ancestor."

Image courtesy of Wikimedia Commons/T. Michael Keesey





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  1. 1. skepticalman 4:08 pm 05/27/2010

    "It is curious that in a century-old race for superlative hominid fossils on a continent currently populated with African apes, we consistently unearth nearly complete hominid ancestors and have yet to recognize even a small fragment of a bona fide chimpanzee or gorilla ancestor."

    Perhaps if we stopped assuming all of those finds were hominid ancestors and started recognizing them as the chimpanzee or gorilla ancestors they really are, those figures would make more sense…. I’m just saying… Let’s practice some real science that doesn’t start with preconceived notions.

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  2. 2. skepticalman 4:13 pm 05/27/2010

    "It is curious that in a century-old race for superlative hominid fossils on a continent currently populated with African apes, we consistently unearth nearly complete hominid ancestors and have yet to recognize even a small fragment of a bona fide chimpanzee or gorilla ancestor."

    Perhaps if we stopped assuming all of those finds were hominid ancestors and started recognizing them as the chimpanzee or gorilla ancestors they really are, those figures would make more sense…. I’m just saying… Let’s practice some real science that doesn’t start with preconceived notions.

    Link to this
  3. 3. rkeolog 5:45 pm 05/27/2010

    Considering the gradualistic development of bipedalism, perhaps paleozoologists and paleoanthropologists should collaborate on the morphological description of bipedal primates. An angle of articulation of C-1 and the foramen magnum or some similar characteristic might be able to establish a preferred mobility template.

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  4. 4. JamesDavis 7:16 am 05/28/2010

    These people loose their credibility when they refer to these animals as human. If you cannot mate with it and produce offspring, then you are not related to it. If you do not believe that theory, then go and try to make a biological family with a chimp, or any member of the monkey family, and let us see a photo of your child…then you will convenience me that we sprang from a member of the monkey family and our DNA is still compatible.

    I think it would be wiser to call a human a human and start calling us human when you found the first modern human skull and call these animals that could walk on two legs an animal and place them in their own category and stop trying to place them with other animals that they are not compatible with. It is impossible for every animal on this planet to had evolved from a single cell. There are billions of cells on this planet and we all evolved from our own cell. We are not the first bipedal animal to walk this planet and we will not be the last. The next bipedal animal that walks this planet will not be related to us, even though they look somewhat like us, does not mean that they will be related to us because they would have evolved from their own cell that contains DNA that will be exclusively theirs that will determine their place on this planet. Unless their DNA is exactly like ours will they have evolved from us.

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  5. 5. dling 4:31 pm 05/28/2010

    It is reasonable to place Ardipithicus on the branch toward the Australopithicines and eventually Hominids, if the evidence is clear that, as rkeolog has stated, that C-1 articulates with the skull in a way that clearly indicates the neck was below the head’s center of gravity. However, I don’t think it is known whether bipedalism was gradual or not, or ratyher, exactly how gradual it was. I would venture that bipedalism may have begun quite rapidly, with the shoulders directly below the head within the course of perhaps only one or two species. On the other hand, ever since Ardipithicus was discovered there has always been some skepticism, as is normal and expected in good scientific practice, regarding whether this species was bipedal. My understanding is that most archaeoanthropologists have accepted the view that Adipithicus was bipedal, but because we don’t have a perfect skull, or yet have a full skeleton showing, say, the hip or leg bones, we are not as sure as we are with subsequent species – the other 20 or so fully bipedal species that we know walked upright.

    And a reminder to JamesDavis, that based on modern DNA analysis and the "molecular clock," that chimps, both species, are much more closely related to Homo sapiens than any of the apes. Thus, they are clearly on our branch, above the split with the apes. This means that from an evolutionarly perspective, they really are not apes any more than we are. Science really needs to address this issue, but part of the problem is figuring out exactly where (or with "who") the split in fact occured, so that we can come up with a relevant and correct name for that early branch – and put chimps in a new and more correct family and genus which is clearly also part of our lineage. Of course this is not to say we evolved from chimps, but from a common ancestor, which very well be Ardipithicus.

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  6. 6. Dimitris 6:42 pm 05/28/2010

    @ JamesDavis

    Please, do some reading of basic biology, or at least try a bit harder to understand this article. There are so many ideas in your comment that are wrong, that it is not even worth debunking.

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  7. 7. jgrosay 11:01 am 05/31/2010

    An updated remake of the Bible note about the first man, Adam, and the birth of Eve, the first woman: Adam was the mutation founder, one and only for a while. The Bible expression: from mud, means from something previously existing in nature, for example an ape. The expression: from a rib of Adam, means just from the flesh of Adam, for example his semen. While he was asleep, a female primate on his herd had sex with him, then giving birth to Eve, Eve being nubile not many years after. Interpret as it is readings are nothing but dark. Salut +

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  8. 8. Dimitris 11:47 am 05/31/2010

    Or simply, the bible is just the mythical imagination of a stone age tribe that tried to make sense of a world they could not understand. I wonder, why is that tribal myth more valid than the ones from Greece or Egypt of the same period? After all, they were both more advanced than the nomad Hebrews of the time.

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  9. 9. gamt67 11:17 am 06/2/2010

    I agree with skepticalman. Furthermore, i’ve been arguing this since grad school 15 years ago; we (many anthropologist) start from the premise, "how do we ‘see’ human traits in this fossil". We don’t find as many chimp fossils as we do hominid, i think that is a valid reseach question.

    As for Mr. Davis, you should probabl read a great deal more about paleogenetics before you comment again.

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  10. 10. mjknoxville 11:40 am 06/7/2010

    "Or simply, the bible is just the mythical imagination of a stone age tribe that tried to make sense of a world they could not understand. I wonder, why is that tribal myth more valid than the ones from Greece or Egypt of the same period? After all, they were both more advanced than the nomad Hebrews of the time"
    Dimitris, can I recommend you read more about how the Bible came to be. Your statement shows your bias and lack of research.

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  11. 11. Dimitris 7:24 pm 06/7/2010

    @mjknoxville

    How did the bible come to be? It is just the creation myth of a neolithic tribe, similar in context and content to the other creation myths of the eastern Mediterranean and influenced by the same environmental factors. I don’t blame them, this was their best attempt to interpret a world that was really inexplicable to them. But if you choose to still follow a neolithic mythical story, why not pick one from a civilisation that was more advanced at the time? They also have mighty sky gods, vengeful adversaries, access to forbidden knowledge, tribes that are divinely selected etc.

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