http://phys.org/news/2012-06-evidence-asia-africa-source-earliest.html
More evidence for Asia, not
Africa, as the source of earliest anthropoid primates
June 4, 2012
An international team of researchers has announced
the discovery of Afrasia djijidae, a new fossil primate from Myanmar
that illuminates a critical step in the evolution of early anthropoidsthe
group that includes humans, apes, and monkeys. The 37-million-year-old
Afrasia closely resembles another early anthropoid, Afrotarsius
libycus, recently discovered at a site of similar age in the Sahara
Desert of Libya. The close similarity between Afrasia and Afrotarsius
indicates that early anthropoids colonized Africa only shortly
before the time when these animals lived. The colonization of
Africa by early anthropoids was a pivotal step in primate and
human evolution, because it set the stage for the later evolution
of more advanced apes and humans there. The scientific paper describing
the discovery appears today in Proceedings of the National Academy
of Sciences.
For decades, scientists thought that anthropoid
evolution was rooted in Africa. However, more recent fossil discoveries
in China, Myanmar, and other Asian countries have rapidly altered
scientific opinion about where this group of distant human ancestors
first evolved. Afrasia is the latest in a series of fossil discoveries
that are overturning the concept of Africa as the starting point
for anthropoid primate evolution.
"Not only does Afrasia help seal the case that
anthropoids first evolved in Asia, it also tells us when our anthropoid
ancestors first made their way to Africa, where they continued
to evolve into apes and humans," says Chris Beard, Carnegie
Museum of Natural History paleontologist and member of the discovery
team that also included researchers from Myanmar, Thailand, and
France. Beard is renowned for his extensive work on primate evolution
and anthropoid origins. "Afrasia is a game-changer because
for the first time it signals when our distant ancestors initially
colonized Africa. If this ancient migration had never taken place,
we wouldn't be here talking about it."
Timing is everything
Paleontologists have been divided over exactly how
and when early Asian anthropoids made their way from Asia to Africa.
The trip could not have been easy, because a more extensive version
of the modern Mediterranean Sea called the Tethys Sea separated
Africa from Eurasia at that time. While the discovery of Afrasia
does not solve the exact route early anthropoids followed in reaching
Africa, it does suggest that the colonization event occurred relatively
recently, only shortly before the first anthropoid fossils are
found in the African fossil record.
Myanmar's 37-million-year-old Afrasia is remarkable
in that its teeth closely resemble those of Afrotarsius libycus,
a North African primate dating to about the same time. The four
known teeth of Afrasia were recovered after six years of sifting
through tons of sediment near Nyaungpinle in central Myanmar.
This locality occurs in the middle Eocene Pondaung Formation,
where the same international research team discovered Ganlea megacanina,
an influential fossil described in 2009 that helped solidify the
presence of early anthropoid primates in Asia.
Details of tooth shape in the Asian Afrasia and
the North African Afrotarsius fossils indicate that these animals
probably ate insects. The size of their teeth suggests that in
life these animals weighed around 3.5 ounces (100 g), roughly
the size of a modern tarsier.
Because of the complicated structure of mammalian
teeth, paleontologists often use them as fingerprints to reconstruct
how extinct species are related to each other and their modern
relatives. These similarities provide strong evidence that Afrasia's
Asian cousins colonized North Africa only shortly before the appearance
of Afrotarsius in the African fossil record. If Asian anthropoids
had arrived in North Africa earlier, there would have been time
for more differences to evolve between Afrasia and Afrotarsius.
The close similarity in age and anatomy shared by the two species
makes Afrasia a touchstone in the quest to date the spread of
anthropoid primates from Asia to Africa.
"For years we thought the African fossil record
was simply bad," says Professor Jean-Jacques Jaeger of the
University of Poitiers in France, the team leader and a Carnegie
Museum research associate. "The fact that such similar anthropoids
lived at the same time in Myanmar and Libya suggests that the
gap in early African anthropoid evolution is actually real. Anthropoids
didn't arrive in Africa until right before we find their fossils
in Libya."
Implications for future research
The search for the origin of early anthropoidsand,
by extension, early human ancestorsis a focal point of modern
paleoanthropology. The discovery of Afrasia shows that one lineage
of early anthropoids colonized Africa around 37 million years
ago, but the diversity of early anthropoids known from the Libyan
site that produced Afrotarsius libycus hints that the true picture
was more complicated. These other Libyan fossil anthropoids may
be the descendants of one or more additional Asian colonists,
because they don't appear to be specially related to Afrasia and
Afrotarsius. Fossil evidence of evolutionary divergencewhen
a species divides to create new lineagesis critical data
for researchers in evolution. The groundbreaking discovery of
the relationship between Asia's Afrasia and North Africa's Afrotarsius
is an important benchmark for pinpointing the date at which Asian
anthropoids colonized Africa.
"Groundbreaking research like this underscores
the vitality of modern natural history museums," says Sam
Taylor, director of Carnegie Museum of Natural History. "Research
like this can only be sustained by the irreplaceable collections,
curatorial expertise, and scientific infrastructure that natural
history museums provide. At the same time, cutting-edge science
like this revitalizes our museum's educational programs and propels
its mission."
"Reconstructing events like the colonization
of Africa by early anthropoids is a lot like solving a very cold
case file," says Beard. "Afrasia may not be the anthropoid
who actually committed the act, but it is definitely on our short
list of prime suspects."
More information: A new middle Eocene primate from Myanmar
and the initial anthropoid colonization of Africa, by Yaowalak
Chaimanee et al. PNAS, 2012.
Journal reference: Proceedings
of the National Academy of Sciences