Higher primates such as humans are considered the brainiacs of the
mammalian world. But a 29-million-year-old fossilized skull
suggests that one of our remote ancestors was a bit of a pea
brain, sporting a noggin smaller than that of a modern
lemur.
The skull belonged to a common ancestor of humans, monkeys
and apes.
"This means the big-brained monkeys and apes developed their large
brains at a later point in time, said lead study author Elwyn
Simons, a Duke University primatologist.
Until now, scientists had assumed brain size was a key feature that
defined higher primates, a category that includes humans, monkeys
and apes. The larger brain relative to body size also has provided
paleoanthropologists with a physical marker for the evolutionary
distinction between higher and lower primates, which include lemurs
of Madagascar.
Tiny enough to fit into the palm of your hand, the skull comes from
a female Aegyptopithecus zeuxis, which means linking Egyptian
ape. This early monkey lived about 33 million years ago, a time
when primates were evolving rapidly. The cat-size primate ate
fruits and leaves in a tropical rainforest in what is now the Fayum
in Egypt.
The discovery, published online this week in the journal
Proceedings of the
National Academy of Sciences, sheds light on the evolution of
human-like brains .
The reason Aegyptopithecus is so important is that its at the
base of the family tree of the Old World higher primates, the group
that were in, Simons told LiveScience. So this is telling us
something about the chapter in our own ancestry.
Small minded
Simons and his team dug up the skull in 2004 from a quarry called
the Jebel Qatrani Formation in Egypt. The cranium was so well
preserved the team used micro computed-tomography (CT) scanning,
which relies on X-rays, to recreate the inside of the skull and
calculate dimensions of the brain it once encased.
He had found a similar, but fragmented skull at the site in 1966.
Comparing dimensions of the old and new skull suggests the 1996
specimen belonged to a male, while the new skull was that of a
female. The size of the female skull suggests it had a brain that
might have been even smaller than that of a modern lemur's," Simons
said.
The new skull also suggests the species had a much smaller brain
than was previously estimated based on the 1996 skull.
Its a little surprising to find out that the brain volume on this
thing indicates that maybe the New and Old World monkeys, their
common ancestor had a prosimian-, or lower-primate-like brain like
Aegyptopithecus does, Simons said.
Sex differences
The team estimates the female weighed about five and a half pounds,
or half the weight of the male. This size difference between males
and females, called sexual dimorphism, is comparable to that in
gorillas, whose genes make them our second-closest relatives next
to chimpanzees.
The stark size difference indicates the monkey-like animals were
social and hung out in multi-male and multi-female troops of 15 to
20 individuals. If we infer that an Aegyptopithecus had a large
social group, that suggests it had enough sense to tell all of
those members apart from nonmembers, Simons said.
"But other features in these skulls, and in many other
Aegyptopithecus fossil pieces collected at the Egyptian site over
four decades, suggest that this primate was already branching away
from its lemur-like ancestry," he said.
Other features also point to an evolving primate. The skull shows
features similar to other higher primates, including a developed
visual cortex, suggesting Aegyptopithecus had acute vision. "So the
visual sense, which is regarded as a very important feature of
anthropoids, or higher primates, had already expanded, Simons
said.
And unlike the prosimians, which run around at night, the animal
had small eye sockets and was likely diurnal (awake during
daylight) like modern and ancient higher primates.
