A new paper in the journal Nature, authored by paleoanthropologists Bernard Wood and Terry Harrison and titled “The evolutionary context of the first hominins,” is critical of the claim that Ardipithecus ramidus (“Ardi”) was a bipedal ancestor of modern humans.
In 2009, the journal Science devoted an entire issue to introducing Ardi and promoting it as a likely human ancestor. It seems that Nature is in something of a rivalry with Science now that it has published this authoritative paper.
When constructing phylogenetic trees, evolutionary biologists generally seek to minimize homoplasy, or instances of convergent evolution. Wood and Harrison observe that the problem for Ardi is that if she is on the human line, then there must be high levels of homoplasy in the alleged hominin tree:
If Ardipithecus is in fact not a hominin then it would require (as noted by its supporters) the confluence of a number of shared specializations developed in parallel between Ardipithecus and later hominins, but the opposite scenario, in which Ardipithecus is assumed to be a hominin, requires remarkably high levels of homoplasy among extant great apes.
A ScienceDaily news release on the paper explains why the case for Ardi as a human ancestor requires inconsistent logic:
For example, the authors claim that for Ardipithecus to be a human ancestor, one must assume that homoplasy does not exist in our lineage, but is common in the lineages closest to ours. The authors suggest there are a number of potential interpretations of these fossils and that being a human ancestor is by no means the simplest, or most parsimonious explanation.
But what if Ardi walked upright? Surely that would qualify her as a human ancestor, right? Not necessarily. Wood and Harrison observe that even features that might normally place a fossil on the human line are known to exist in ape-like species that were not ancestral to humans. The same news release explains this problem well:
Oreopithecus bambolii, a fossil ape from Italy shares many similarities with early human ancestors, including features of the skeleton that suggest that it may have been well adapted for walking on two legs. However, the authors observe, enough is known of its anatomy to show that it is a fossil ape that is only distantly related to humans, and that it acquired many “human-like” features in parallel.”
This argument isn’t new. In 1998, Christopher Wills (who taught human evolution for an evolutionary biology course I took in college) wrote: “Upright posture may not be unique to our own lineage. An ape that lived ten million years ago on Sardinia, Oreopithecus bambolii, seems to have acquired similar capabilities, perhaps independently.” (Children of Prometheus, p. 156) Wood and Harrison’s article now elaborates on the dangers of assuming that even an “impressive suite of shared features with fossil hominins” found in Oreopithecus ought to imply a close relationship to humans:
What is instructive about Oreopithecus with respect to developing hypotheses about the relationships of Ar. ramidus is that it is a species of hominoid that is well-enough known anatomically (that is, almost every bone in the skeleton is represented) to be certain that it is not a member of the hominin clade, yet it shares many anatomical similarities with later hominins, including some that are generally considered to be uniquely associated with bipedal behaviour. The shared similarities include: small and vertically implanted incisors, relatively small canines, a small non-sectorial P3 with a high incidence of a prominent metaconid, absence or small size of a diastema in the upper tooth row, a vertically oriented mandibular symphysis, a mental foramen situated high on the mandibular corpus, a short orthognathic face, an anteriorly placed zygomatic process of the maxilla, anterior projecting nasal apophyses and a deep pit on the palmar aspect of the terminal phalanx of the thumb for attachment of a well developed flexor pollicis longus tendon. Shared similarities associated with bipedal behaviour include an anteriorly situated foramen magnum, short and broad iliac blades, infero-superiorly short pubic symphysis, a well-developed anterior inferior iliac spine, a large ischial spine, medial and lateral condyles of the distal femur similar in size, possibly associated with a bicondylar angle. The impressive suite of shared features with fossil hominins led Hürzeler to deduce (not unreasonably) that Oreopithecus was a fossil hominin, but these features are most parsimoniously interpreted as either homoplasies or retained primitive hominid features. Oreopithecus is a classic example of how a late Miocene hominid can independently acquire a suite of structural- functional complexes of the dentition, cranium, hand, hip and hindlimb that closely parallel the specialized features uniquely associated with the hominin lineage, and thereby encourage researchers to generate erroneous assumptions about evolutionary relationships. Oreopithecus highlights the dangers inherent in uncritically assuming that shared similarities are a secure indication of relationship or that extant primates are an adequate guide to the potential behavioural diversity of extinct taxa.
The object lesson that Oreopithecus provides is critical to the debate about interpreting the relationships of the earliest purported hominins. It demonstrates how features considered to be hominin specializations can be shown to have been acquired independently in a non-hominin lineage in association with inferred behaviours that are functionally related to, but not necessarily narrowly restricted to, terrestrial bipedalism.
(Bernard Wood & Terry Harrison, “The evolutionary context of the first hominins,” Nature, Vol. 470:347-352 (Feb. 17, 2011).)
The authors urge that more caution is needed when claiming that Ardi is a human ancestor:
We do, however, advocate that those palaeoanthropologists whose considerable and much valued efforts in the field are rewarded with fossils as significant as those from Aramis, Toros Menalla, Lukeino and Malapa acknowledge the potential shortcomings of their data when it comes to generating hypotheses about relationships. We urge researchers, teachers and students to consider the published phylogenetic interpretations of these taxa as among a number of possible interpretations of the evidence.
That’s a nice way of saying that Ardi’s initial promoters pushed the case for Ardi’s status as a human ancestor further than was warranted by the data.
This isn’t the first time that Ardi’s status as a human ancestor has been challenged. As we reported last year, Time Magazine reported that paleoanthropologist Esteban E. Sarmiento “regards the hype around Ardi to have been overblown.” Hopefully this well-argued paper by Wood and Harrison will encourage further caution and restraint among evolutionary paleoanthropologists eager to overstate claims of fossil human ancestors when speaking to the public.