IT WAS in the dust of an industrial limestone quarry near Shanghai in China that palaeontologist Chris Beard made the discovery of a lifetime. It didn鈥檛 look like much 鈥� just a few small teeth set in a fragment of lower jaw 鈥� but it changed his view of human evolution forever. By naming the fossilised remains Eosimias sinensis, or 鈥渄awn monkey of China鈥�, he knew he was courting controversy. He got it.
Beard, of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, had committed two scientific heresies. First, he was implicitly claiming to have found the first simian, or higher primate, and therefore the oldest known direct ancestor of humans. Secondly, he was implying that our early ancestors came from Asia.
These claims could hardly be further from the orthodox view, which is that the higher primates evolved in Africa about 35 million years ago, diversified in Africa, and did not spread out into Asia until 10 million years later. In other words, the entire story of human evolution was played out in Africa. But to Beard and his supporters, the discovery of Eosimias suggested something different: the higher primates evolved in Asia, diversified in Asia and only later spread to Africa.
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In appearance at least, Eosimias would seem to bear little relation to a human. From jaw and other bone fragments, Beard has been able to reconstruct its likely appearance and make some guesses about its behaviour. 鈥淚t would be small enough to hold in the palm of your hand,鈥� he says. 鈥淚t would have a fast metabolism, eating high-calorie foods like insects and fruit and have nervous, almost frenetic, behaviour.鈥� Like other primates Eosimias would have forward-looking eyes and grasping limbs. Its closest living analogue is probably the pygmy marmoset.
But although it might not look much like a modern higher primate, particularly apes and humans, Beard believes Eosimias is a vital link in the story of how humans came to be. 鈥淭he evolution of higher primates has immense scientific and philosophical implications for human origins,鈥� he says. 鈥淭here has been an intense focus on the last little segment of human evolution, but imagine trying to document the history of western civilisation if we knew nothing about Plato, Aristotle or the rise and fall of the Roman empire.鈥�
Higher primates, also known as simians or anthropoids, include new and old-world monkeys, apes and hominids. The rest of the primate order, called lower primates or prosimians, comprise the lorises, bushbabies, lemurs, tarsiers and two extinct groups (see Diagram). There are around 240 living species of primate ranging from the mouse lemur to the 160-kilogram mountain gorilla. All share stereoscopic colour vision, grasping limbs and larger brains than other mammals. What distinguishes higher primates from the prosimians is a greater reliance on sight compared with the other senses, flexibility of movement culminating in bipedalism and, crucially, larger brains.
The higher primate suborder is diverse, ranging from squirrel monkeys to baboons, chimpanzees to humans. But at some point in the past we all shared a common ancestor. The general consensus is that this ancestor split off from the Tarsiidae, a group of insect-eaters that lives on in the form of the saucer-eyed tarsiers, about 30 million years ago.
Until recently there seemed little doubt that this split occurred in Africa. During a dig at the famous Fayum deposits in Egypt in 1965, Elwyn Simons of Yale University unearthed a fossil that was undisputedly a higher primate. He named the cat-sized creature Aegyptopithecus (pithikos is the Greek word for ape) and dated it as living around 33 million years ago, near the beginning of the Oligocene.
Fayum has since yielded even older fossils of what are almost certainly higher primates. The best known is probably Catopithecus browni. In 1992 Simons discovered several near-complete skeletons of this creature dating from around 35.5 million years, the late Eocene (Science, vol 268, p 1885). Like Eosimias, Catopithecus was a small, insect-eating quadruped. It was most likely tree living, and had a small brain and long snout. Close examination of the teeth and skull clearly mark the creature out as a higher primate.
No respected palaeontologist disputes that Catopithecus is a higher primate, but in the past decade new fossils have emerged that have shaken the evolutionary tree to its roots. Some are from North Africa. But the most controversial are from China, Burma and Thailand, including Beard鈥檚 Eosimias.
The first hints that Asia played a part in the story came in 1927 with the discovery of a few fossil fragments in mid-Eocene deposits in Burma. Named Pondaungia cotteri, it was almost certainly a primate of some kind, possibly a higher primate. But the remains were too sparse for anyone to be sure which branch of the family tree it belonged to. Ten years later another fossil of the same age was uncovered. Amphipithecus looked even more like a higher primate, but again the remains were too fragmented to say for certain.
It wasn鈥檛 until 1994 that the publication of Beard鈥檚 discovery put Asia on the map (Nature, vol 368, p 604). In his paper he described five new species of primate dating from around 45 million years ago, the middle of the Eocene, long before the higher primates were thought to have evolved. Four were clearly prosimian but one species stood out. 鈥淚t was definitely a eureka moment,鈥� Beard recalls, 鈥淚 was in the laboratory back in Beijing looking at jaw fragments under a microscope when I decided to check them against a standard reference text.鈥� The comparison, he says, was startling. 鈥淭he lab erupted. We all started yelling and clapping, it was so exciting.鈥�
If he was right then Eosimias sinensis would have been the oldest known higher primate, predating the earliest Fayum fossils by 10 million years.
But the remains were too fragmentary for his enthusiasm to be shared by many of his peers. His claim was not so much met with scepticism as with downright derision. Some palaeoanthropologists doubted that Eosimias was a primate at all, let alone a higher primate. In a particularly scathing criticism, one expert suggested Beard had merely stumbled upon some sort of hedgehog.
Two years later Beard came back with more evidence from China, describing a second species of Eosimias, dubbed E. centennicus. It was from slightly younger deposits than Eosimis sinensis, but still predated the African fossils by nearly 10 million years. And this time he had a complete lower jaw in his possession, adding weight to his claim that Eosimias was indeed a higher primate (Science, vol 272, p 82). Then, in 2000, he found ankle bones. 鈥淭hese are highly diagnostic for palaeontologists,鈥� Beard says. 鈥淓very species is unique and in primates the foot is often the centre of specialisation.鈥� To no one鈥檚 great surprise, he interpreted the bones as being distinctly anthropoid.
As a result of Beard鈥檚 finds, attention turned once again to the Burmese fossils. In a series of publications, Jean-Jacques Jaeger of the University of Montpellier II in France reported several lines of evidence that support Beard. Four years ago he claimed the study of a lower jaw from the middle-Eocene Pondaungia confirmed it as an anthropoid (Proceedings of the National Academy of Sciences, vol 97, p 4102). And late last year he described an ankle bone, probably from Amphipithecus, as being 鈥渄ecisive evidence鈥� that the Burmese animals were higher primates (Proceedings of the National Academy of Sciences, vol 100, p 13173). 鈥淭he paper seals the fate for Amphipithecus,鈥� Beard says. 鈥淭heir ankle bones look like a monkey鈥檚. Their teeth look like a monkey鈥檚 too.鈥� Jaeger and his group have also described two more species from the Asian Eocene that they claim are higher primates, one in Burma and another in Thailand. And in 2001 a Japanese team found yet another, also in Burma. It looked as though Asia was teeming with higher primates 45 million years ago.
There is also other, more circumstantial, evidence for Beard鈥檚 claims of an Asian origin. He points out that modern tarsiers, the closest living relatives to the higher primates, are only found in south-east Asia 鈥� in Indonesia and the Philippines to be precise. What is more, their fossil record is also confined to Asia. Beard points out that Darwin argued for a human origin in Africa because chimps are confined to that continent. The same logic applies to the higher primates, he says. For him the evidence is compelling: the higher primates, our direct ancestors, evolved in Asia.
It is fair to say that Beard鈥檚 views are highly controversial. 鈥淭here is no evidence of anthropoid origins in Asia,鈥� says palaeontologist Russell Ciochon of the University of Iowa in Iowa City, who is one of Beard鈥檚 most vocal critics. 鈥淚t鈥檚 a no-brainer 鈥� they evolved in Africa.鈥� And Ciochon reckons he has compelling evidence to back up his statement, because over in Africa a rather different story has been unfolding.
In 1992, Marc Godinot of the University of Montpellier II in France and Mohamed Mahboubi of the University of Oran in Algeria were digging near Zegdou in the Algerian Sahara when they unearthed what they described as the 鈥渆arliest known simian鈥� (Nature, vol 357, p 324). It was, they concluded, a higher primate from the early or middle Eocene, making it at least as old as Eosimias and possibly older. What鈥檚 more, analysis of tooth fragments suggested it was much more evolutionarily advanced than Eosimias. They called it Algeripithecus and claimed it supported the idea of 鈥渁 long and endemic African history for higher primates鈥�. Subsequent discoveries in Oman, Tunisia and Egypt, while younger than Algeripithecus, have confirmed a tremendous diversity of what most researchers agree are higher primates.
Splendid isolation
Surprisingly, Beard does not dispute that Algeripithecus is a higher primate, nor that it pre-dates Eosimias, nor even that it is more advanced. But he says this does not negate his argument. It just means that higher primates had a long evolutionary history in Asia and made their way to Africa millions of years before even Eosimias came along. If he is correct, then the 鈥渄awn monkey鈥� is not the earliest Asian higher primate but is much further down the evolutionary track.
So why does Algeripithecus appear to be more advanced? 鈥淚t makes perfect sense,鈥� Beard says. 鈥淎t an early date, anthropoid primates invaded Africa from Asia where they originated. They encountered a virgin continent, allowing them to evolve and radiate out in splendid isolation.鈥� As a result, he says, African higher primates evolved rapidly, while in Asia there was huge competition from the sheer diversity of prosimians there.
This scenario is not as unlikely as it sounds. In the early Palaeocene, Africa鈥檚 mammals comprised ancestral elephants, hyraxes, elephant shrews, aardvarks, tenrecs and manatees. Many of the groups we think of as quintessentially African, including cats, zebras and rhinos, arrived later from Asia. Why couldn鈥檛 the same apply to higher primates?
Not everyone is convinced. The sheer diversity of Eocene fossils from Africa demonstrates to Simons, who is now at Duke University in Durham, North Carolina, that even if higher primates did not originate in Africa, they had an 鈥渆xtensive earlier history鈥� there. For Ciochon the evidence is more clear-cut. 鈥淚f they are of Asian origin, shouldn鈥檛 these early forms in Africa look like Asian anthropoids? The earliest anthropoids are found in Africa, period.鈥�
Perhaps the best hope of a definitive answer comes from the discovery of new fossil deposits. Gregg Gunnell, a palaeontologist at the University of Michigan in Ann Arbor, for example, has just opened one up in Tanzania which is contemporaneous with Algeripithecus. The site is an ancient lake bed, which is a good preserver of mammals. Gunnell has already described a species of bat he found there. If higher primates were all over Africa at that time, then the lake bed may well contain their remains.
In the meantime, Africa proponents, notably Ciochon, Simons and Gunnell, decided to re-examine the Asian primates for themselves. They began by spending almost a month in Burma studying Pondaungia. Working long hours seven days a week, they returned to publish a series of papers and articles. Their analysis of bones and teeth was conclusive: 鈥淚t does not support the inclusion of Pondaungia in Anthropoidea鈥� (Proceedings of the National Academy of Sciences, vol 98, p 7672). Instead they decided the animal was a member of an extinct group of prosimians known as the adapids. They argue that any similarity with the higher primates is purely coincidental. It arises, they say, from convergent evolution as a result of a common diet of nuts, seeds and unripe fruit.
Since then Ciochon and Gunnell have re-examined the other Asian finds, and conclude that none are higher primates. They suggest instead that simians did not reach Asia until well into the late Oligocene, around 25 million years ago 鈥� 20 million years after Eosimias. As for Eosimias itself, they reckon it too is a prosimian, which bears a passing resemblance to a true anthropoid by virtue of convergent evolution. And the 鈥渄ecisive鈥� ankle bone is anything but 鈥� Jaeger鈥檚 group cannot even say for sure which species it comes from.
There is also another problem with the 鈥渙ut of Asia鈥� theory and it鈥檚 not one of anatomy but geography. In the Eocene, the world looked very different. 鈥淭here was a huge sea,鈥� Ciochon says bluntly. 鈥淗ow could Asian primates get to Africa? The answer is they didn鈥檛.鈥�
Beard admits that crossing hundreds of kilometres of ocean would have been difficult, but says the feat is by no means impossible. At the time the Indian subcontinent was an island that had yet to collide with Asia. He reasons that this could have acted as a giant stepping stone for primates between the Asian and African continents. Alternatively, he suggests, the animals could have used similar blocks of land between Asia and north Africa across what was then another body of sea. 鈥淭his was a lucky event,鈥� Beard admits, 鈥渂ut it鈥檚 the reason we鈥檙e here and able to talk about it.鈥�
Despite the detractors, Asia proponents are convinced that their theories stand up to scrutiny. All have been published in respected journals and there is good evidence that Eosimias, in particular, has anthropoid characteristics. If higher primates did indeed evolve in Asia and somehow make it to Africa, Beard says it would have been 鈥渓ike a vast empty garden of Eden. Africa was the Petri dish for anthropoids to get in, evolve like crazy and eventually take over the world.鈥�
If Beard鈥檚 scenario is right, the higher primate lineage owes its amazing evolutionary journey to the fact that a primitive simian dropped, purely by chance, into the unoccupied verdant paradise of Africa. From our human perspective this would surely be one of the most amazing evolutionary events ever. Just imagine, says Beard, if a lemur had got there first.