The Oldest Primate On The Planet

The beautiful, 47 million year old fossil of
Darwinius masillae, better known as Ida

When the fossil of Darwinius masillae, better known as Ida, was first discovered in 2008 in the Messel Pits, Germany, it caused a massive media stir. Jorn Hurum, the Swedish palaeontologist who described Ida in his 2009 paper, claimed that Darwinius was the common ancestor of both the two major groups of primates, the haplorhines (simians such as tarsiers, monkeys and humans) and the strepsirrhines (lemurs and lorises), making it what was considered to be the the discovery of the year.

If Hurum was correct, then Ida was not only the discovery of the year, but one of the the most important finds in palaeontological history. Yet skeptics gathered like vultures. It quickly became clear that Darwinius was more closely related to strepsirrhines than haplorhines and as a result could not be the common ancestor of both groups.

Ida is now believed to lie very close to the point of divergence. At 47 million years old, the common ancestor was probably around 50 million years old. Now a fossil discovery of a primate from China has shown that the split occurred further back than palaeontologists first thought. Around a decade ago, a farmer living in the Hubei Province of China uncovered a fossil from a rock formation close to the Yangtze River. The tiny, but well preserved remains embedded in a thin slab of dark grey slate were taken to the European Synchrotron Radiation Facility (ESRF) for analysis.

The fossil from near the Yangtze River in the Hubei Province of China

A synchrotron is a particle accelerator which fires electrons around a tunnel until they are close to the speed of light. At this point, they emit powerful x-rays which are focused onto a sample, to reveal its hidden features, allowing it to be reconstructed in detail. The parts of the skeleton still obscured by slate were brought into view. 'There's no way you can prepare the fossil any better to see its features,' explained Dr Paul Tafforeau from the ESRF. 'This gives you access to the general anatomy and we can achieve very high resolution.'

An artist's impression of Archicebus achilles in an ancient tropical forest.
Overall, it would have been about the size of a mouse

With its long legs and tail and grasping, prehensile feet, it was clearly arboreal and based on its small, pointed teeth, most likely fed on insects which would have been more than enough to satisfy the daily calorific need of a creature its size. Its importance, however, lies with its relationship to other primates. At just 7 centimetres in length and 30 grams in weight, it is the smallest known member of the group on Earth and the most primitive to date, coming in at an incredible 55 million years old, making it older than both Ida and the proposed point of divergence between the strepsirrhines and the haplorhines.

In response to this, the researchers gave it the name of Archicebus achilles which translates as 'ancient monkey.' While the general physiology with its small frame and long tail was similar to that of a lemur, its calcaneus (heel bone) showed without a shadow of a doubt that it was more closely related to the tarsiers and therefore monkeys and humans, a detail referenced by the species name of achilles.

'The heel, and the foot in general, was one of the most shocking parts of the anatomy of this fossil,' said Dr Chris Beard from the Carnegie Museum of Natural History, Pittsburgh, USA, 'because, frankly, the foot of this fossil primate looks like a small monkey, specifically like a marmoset.' What this tells us is that the split in the primate family must have occurred, and therefore the common ancestor itself lived, more than 55 million years ago.

A cladogram showing the position of Archicebus within the
primate group. Ida occupies the now extinct adapid branch. The
lower half of the phylogram comprises the haplorhines while the
upper half comprises the cladogram.

As Archicebus  is also more closely related to tarsiers rather than displaying the perfect mix of characteristics, which would make it the common ancestor of tarsiers and their sister group the anthropoids (the rest of the haplorhine group such as monkeys and humans), this shows that the first split in the primate family must be older still, perhaps occurring more than 60 million years ago. The dinosaurs died out 65 million years ago due to a meteorite impacting the Earth. The planet, while blighted, only remained a harsh place to live for a million years at the very most.

When plant life reclaimed its surface, the surviving animals followed in their wake. The dinosaurs were gone, leaving behind thousands of exploitable ecological niches. Tropical forests sprang up across the planet, hotbeds of evolutionary activity, in which the first, yet still undiscovered, primate evolved, giving rise to a diverse wave of descendants, including Archicebus and Darwinius.

Both creatures, based on their geological settings inhabited similar environments. Both were arboreal jungle-dwellers as seen in the plant fossil record at each locality. Ida was found in a piece of shale which originally formed on the bed of a lake. Archicebus, which has yet to receive a moniker of its own, was also found on a piece of slate, originally formed on the bed of a lake. Yet while the habitats were similar, they were separated by tens of thousands of miles.

The configuration of the continents barely changed between 55 and 47 million years ago, meaning that the ancient lakes in Hubei and Germany would have been tens of thousands of miles apart, as they are today. The fact that the older Archicebus was discovered in Asia rather than in Europe where the younger Darwinius made its home provides strong evidence that primates first evolved in the far east rather than in Africa or Europe.

Previously, the only fossil evidence backing this hypothesis were a few teeth from Myanmar. This discovery on the other hand is of a complete and distinct species with clear physiological links placing it close to the base of the primate family tree. In my opinion, the theory is most likely correct. Not only does the fossil evidence strongly point to this being the case, but the theory itself provides a solution to one of the major problems facing palaeontologists: how did monkeys reach the Americas?

The haplorhine family is divided into two groups: the Old World and New World monkeys, or catarrhines and platyrrhines to use the Latin names. The former inhabit Africa and Eurasia (the old world) while the latter live in the Americas (the new world). Previously, the fossil record suggested that the first primates evolved in Europe/Africa and gave rise to the catarrhines around 40 million years ago. The platyrrhines in turn evolved from Old World monkeys which somehow migrated across the Atlantic ocean.

Various theories were put forward to suggest this, including monkeys carried across on chunks of mangrove swamp broken off from the African coast by storms and pushed towards the Americas by the currents. Irrespective of the method, an ocean crossing would have been incredibly dangerous and unlikely to occur. Instead, if primates first evolved in Asia and split into the strepsirrhines and the haplorhines (composed of the catarrhines and the platyrrhines), the catarrhines could have simply migrated towards Europe and Africa.

A map showing how Asia and the Americas were originally connected by a
land bridge known as Beringia. The blue line indicates the path the
 platyrrhines may have taken to reach the new world

Meanwhile the platyrrhines could have crossed the land bridge connecting Asia and the Americas in order to reach the new world, thus cutting out the need for a lengthy and hazardous sea crossing. While the land bridge today is submerged beneath the Bering Sea, up until just 6000 years ago, it allowed for an easy crossing between the two continents. What is more, the oldest platyrrhine fossil, a monkey known as Branisella which was found in Bolivia, is just 26 million years old, giving primates over 10 million years to make the move from Asia to the Americas based on the oldest fossils of both haplorhine groups.

Of course, this is a lot to place on a single, mouse-sized fossil, which is why palaeontologists have been conducting detailed analyses of Archicebus for more than ten years. As a result, its placement on the primate family tree and the theories as to its impact on the group's evolutionary history can be considered accurate. 'We applied rigorous testing to all our ideas and hypotheses, convincing all our own collaborators first,' said John. J. Flynn, the curator of fossil mammals at the American Museum of Natural History.

This is in stark contrast to the discovery of Ida who was proclaimed the common ancestor and missing link in our evolutionary story barely a year after her remains were purchased. The past few years have been tantalising. While she was discarded as the common ancestor, Ida showed us that we were near to the base of the primate family tree. Archicebus has brought us closer still and while we continue to claw our way back through millions of years, I believe that the Discovery will be made sooner rather than later.

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For those of you who are interested, the article on
the first piece of fossil evidence backing up the theory of the Asian origins of primates can be found by following the link below:

http://prehistoricict.blogspot.co.uk/2012/06/revolution-in-primate-prehistory.html

Enjoy!