Humanity’s earliest cousin

55 million-year-old Archicebus is the oldest primate skeleton found, the closest we’ve come to discovering our origins, and suggests we evolved in Asia.

Our distant ancestors evolved not in Africa but Asia, in a hothouse world newly free of dinosaurs. Over 55 million years ago, in the lush rainforests of what is now east Asia, a new voice was heard in the animal chorus: the cry of the first primate.

A fossil unveiled this week might give us an idea of what this crucial ancestor looked like. It is the earliest primate skeleton ever found. It also strongly suggests that our lineage evolved in Asia, several million years earlier than we thought, and links the evolution of primates to the most extreme episode of climate change of the last 65 million years.

“For the first time we can shine a light on this critical part of the [evolutionary] tree and say what did these animals look like?” says Christopher Beard at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. The find, says Erik Seiffert of Stony Brook University in New York, who was not involved in the study, is “undoubtedly one of the most important discoveries in the history of palaeoprimatology”.

Beard and his colleagues found Archicebus achilles in eastern China, just south of the Yangtze river (Nature, DOI: 10.1038/nature12200). It is 55 million years old, has the relatively small eyes of an animal active during the day and the sharp molar teeth of an insect-eater. Significantly, it also has the hindlimbs and flexible foot of a primate that had already taken to leaping between branches and gripping onto them with its feet – characteristics that we only lost when our ancestors left the trees just a few million years ago. In fact, a recent study revealed that at least 1 in 13 of us still has a flexible foot (New Scientist, 1 June, p 11): the trait, it seems, may trace all the way back to an animal very like Archicebus.

At the moment, analysis of Archicebus places it not on our direct line, but with our next-door neighbours, the tarsiers of south-east Asia. “But it is incredibly close to the junction, and it wouldn’t take much to flip it over to our side of the tree,” says Beard. “I wouldn’t be totally shocked if in future we were to find out that Archicebus is a basal anthropoid [the group comprising monkeys, apes and humans].”

He has one main reason for believing Archicebus is closer to home than he can prove at present. Parts of its body are eerily similar to what we would expect to find in our oldest ancestor. Its ankle bone, in particular, looks just like a monkey’s – a feature that led the team to name the remarkable fossil after the Greek hero Achilles.

Jean-Jacques Jaeger of the University of Poitiers in France agrees that Archicebus‘s mix of tarsier and anthropoid traits make it hard to say which group it belongs to. It could also turn out to have belonged to a third group that didn’t survive, he adds.

There are no other good primate fossils from this far back in time. Until we can compare it with its peers, says Beard, Archicebus‘s exact place in the tree is going to be unclear. “We’re almost operating in a vacuum here.”

Perhaps most significantly, the new fossil supports the idea that primates originally evolved in south-east Asia, and suggests the ancestors of all monkeys and apes had already split off from other primates 55 million years ago – millions of years earlier than textbooks suggest. “Many of us suspected that was the case,” says Beard. “Archicebus solidifies that this important branching event goes right back to the beginning of the Eocene.”

This links the birth of our primate line to a major spike in global temperatures known as the Palaeocene-Eocene Thermal Maximum (PETM). It also puts our point of origin squarely in the heart of the PETM furnace: equatorial Asia. “Primates are probably the most tropically adapted mammals alive today,” says Beard. It therefore makes sense that they originated in a warm climate.

What’s more, south-east Asia would have offered a refuge for tropical species to weather the storm of cooler times: while Earth’s drifting tectonic plates dragged all major continents across the latitudes, this region remained where it was, right on the equator.

“There were so many ecological niches available in those forests [after the extinction of the dinosaurs 65 million years ago],” says Jaeger. Hot temperatures, he adds, may have provided primates and other mammals with rich environments to expand into. “You get more insects and fruit in an equatorial climate. They benefited from that.”

How, then, did Africa end up being the cradle of humanity? If the out-of-Asia hypothesis is correct, early monkey-like primates must have moved from Asia over to Africa, sometime around 40 million years ago, Beard says. The hypothesis must still explain how they made the move, crossing the vast Tethys Sea which separated Asia and Africa. As Jaeger puts it: “It is difficult to imagine small mammals surviving on a raft for two weeks in the sun.”

However, the extreme heat of the PETM, and the lingering warmth over the next few million years, could have helped, says Thierry Smith of the Royal Belgian Institute of Natural Sciences in Brussels. The high temperatures would have created lush tropical forests that spanned continents. Early tree-dwelling primates could only have spread through such forests. “A tarsier on the ground is prey, because it cannot walk. It’s like a crab, it’s terrible.”

However it happened, the move would prove to be a pivotal moment in our history. “[Primates] underwent a real starburst of evolution in Africa, which eventually led to apes and humans,” says Beard.

We can expect to hear much more about that first colonisation of Africa in the near future. “Humanity’s spread out of Africa was undoubtedly very important, but it wouldn’t have happened if our primate ancestors hadn’t got into Africa in the first place,” says Beard. “The ‘Into Africa’ story is very big right now – and it’s only going to get bigger.”

The elusive ‘missing link’

Since Archicebus is so old, many will wonder if it is a “missing link” – the species that gave rise to all primates alive today. The answer is a confusing mix of “yes”, “no” and “maybe”.

NO Archicebus is probably not the ultimate link in our primate family tree. Finding that animal is remarkably difficult, because the earliest chapters of the primate story have left hardly any good fossils. While Archicebus is an exciting find, it is not an ancestor of lemurs and lorises, as the evolutionary tree above shows. There may be an older, more primitive primate out there waiting to be discovered.

You may remember a 47-million-year-old fossil unveiled in May 2009. Nicknamed Ida, it was touted as this missing link. Most biologists now think it is no more than an early member of the lemur and loris family.

YES Archicebus lies very close to the junction between tarsiers and monkeys, apes and humans. It could be as close as we are ever likely to get to the missing link between these two important groups of primates.

MAYBE For now, Archicebus lies on the tarsier side of the tree. If and when more good fossils from this period are found, it is possible it might flip onto our side (see main story). Until we can fill in more of the picture, though, it’s a bit of a stretch to call it one of our missing links.

(Article written by Colin Barras and Michael Marshall, contributers to New Scientist magazine)

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