the ‘missing link’ disproved?

A friend of mine’s sent me an item from National Geographic with the headline ‘Oldest "human" skeleton found – disproves "missing link"’. (Thanks, Heather!) The story itself is based on the publication this week of a series of papers describing aspects of Ardipithecus ramidus, & they make extremely interesting reading. But before I start talking about them, please allow me a little vent…

I really dislike headlines like this! OK, they attract attention, but… Essentially what it’s saying is that the last common ancestor that humans share with our sister species, chimpanzees, wasn’t particularly chimp-like, so we can kiss the idea of that sort of ‘missing link’ goodbye. But is there any good reason why that last common ancestor might have been like modern chimps? After all, they’ve been progressing down their own evolutionary path over the last 6 million years or so, just like us. The article also notes that ‘Lucy’ aka Australopithecus afarensis is no longer ‘it’. But then, at 3.2 million years old, she was always going to be too young to be even remotely representative of that common ancestor, which DNA analyses suggest lived around 6my ago. Anyway, back to the chase…

Despite all the hype, Ardipithecus ramidus isn’t new to science. The species was first discovered in 1992, and at 4.4my old has long been considered the earliest known confirmed  hominin. (The older Orrorin tugenensis is too fragmentary for scientists to be confident of its relationship to our own lineage, although that’s where its discoverers place it, & it’s the same for Sahelanthropus tchadensis.) Since then, a large number of scientists have been examining not only the fossil remains of this species, but also a range of other materials that have allowed them to take an extremely thorough look at its environment. The result: 11 papers published in the 2 October edition of Science – they’ve been made freely available; you just need to register here to access them. So make no mistake; this is a uniquely detailed presentation of the biology & ecology of an early hominin, a story that’s been nearly 20 years in the making (unlike the rushed presentation of ‘Ida’ to the world!). As the editors of Science say:These 4.4 million year old hominid fossils sit within a critical early part of human evolution, and cast new and sometimes surprising light on the evolution of human limbs and locomotion, the habitats occupied by early hominids, and the nature of our last common ancestor with chimps.

One of those 11 key papers is Ardipithecus ramidus and the palaeobiology of early hominids (White et al. 2009). (It’s accompanied by an ‘authors’ summary’ that outlines the physical features of ramidus & places it in an ecological context – this would be good for students to read as an entry point to the full scientific paper.) The authors have used existing & new material – and among the fossils they used is the partial skeleton of one individual, dubbed ‘Ardi’.

Fig. 3 from White et al. (2009): The ARA-VP-6/500 skeleton. This is a composite photograph to show the approximate placement of elements recovered. Some pieces found separately in the excavation are rejoined here. Intermediate and terminal phalanges are only provisionally allocated to position and side.

Isn’t that lovely? And isn’t it wonderful that so much of this individual has survived for such a long period of time. Fossilisation of terrestrial animals is relatively rare, & finding such fossils can be very difficult & entails a certain amount of luck. As White & his colleagues note, "bones of medium & large mammals were usually ravaged by large carnivores, then embedded in [sediments]. Once exposed by erosion, postdepositional destruction of the fossils by decalcification & fracture is typical."  That was certainly true for the remains of this individual, which were extremely fragile – to the extent that it took several years to restore. Other fossils found in the same strata suggested that ramidus lived in wooded habitats, not open grasslands. This gives food for thought as up until now we’ve thought that early human evolution was linked to the expansion of grasslands.

The skeleton represents the remains of a female who would have been around 120cm tall & weighed about 50kg. Its cranial capacity was small, around 350cc (much the same as Sahelanthropus) & the face was prognathous, although less so than modern great apes. Teeth can tell us a lot about diet, & those of Ar. ramidus suggest that it was omnivorous & didn’t eat particularly hard foods. In addition to wear patterns on the teeth, the scientists used data on carbon isotopes from the teeth of 5 other ramidus individuals to conclude that they ate a relatively wide range of plant foods that didn’t include grasses & other savanna plants.

How did this species get arouind its woodland environment? Look at those feet – there’s a sizable gap between the big toe & the rest of the digits, so the foot would have had some ability to grasp branches.However, White et al. also note that the foot had some ‘propulsive capactiy’ ie the individual could also walk bipedaily. Besides that big toe, the forearm:lower leg ratio (similar to that in macaque monkeys, which run along the top of branches) also suggests that ramidus could get around in the trees as well as walk on the ground, as does the structure of the wrist. But there are no adaptations for knuckle-walking – this is significant as we’ve generally thought that the last common ancestor of chimps & hominins was a knuckle-walker. On this basis the authors infer that the gorilla-chimp-human last common ancestor moved around in trees by walking on the tops of branches, and that hominids have never knuckle-walked. This would mean that knuckle-walking & brachiation (swinging along under branches) evolved after the human lineage split from that of the other great apes. And they conclude that

Perhaps the most critical single implication of Ar. ramidus is its reaffirmation of Darwin’s appreciation: Humans did not evolve from chimpanzees but rather through a series of progenitors starting from a distant common ancestor that once occupied the ancient forests of the African Miocene.

 

As you might expect, there’s been a lot of coverage of this by other bloggers: Carl Zimmer’s "Ardipithecus: we meet at last", "At long last, meet Ardipithecus ramidus" by Brian Switek (who provides a very thorough commentary on Ardi’s remains), & "Ardipithecus ramidus" from PZ Myers, to name a few.

T.D.White, B.Asfaw, Y.Beyene, Y.Haile-Selassie, C.Owen Lovejoy, G.Suwa & G.WoldeGabriel (2009) Ardipithecus ramidus and the palaeobiology of early hominids. Science 326: 64 (authors’ summary) & 75-86. doi 10.1126/science.1175802