Author : Wahid Ahmad
The separation of South America from Africa occurred around 80 million years
ago during the breakup of the supercontinent Gondwana. Around 36 to 40 million
years ago, caviomorph rodents and ancestors of New World Monkeys suddenly
appeared in South America.
In this article, we will untangle this puzzle. How did monkeys
make it to the Americas?
Caviomorph rodents diversified into modern-day capybaras, chinchillas,
viscachas, and New World porcupines. Monkeys in Central and South America
called Platyrrhini or New World monkeys are a diverse group of primates, with
over 170 different species. They make up more than 10 per cent of mammal
species in South America.
These monkeys come in various sizes, from tiny pygmy marmosets to large Ateline
males that can weigh over 10 kilograms. Some fossils even suggest there were
even larger monkeys in the past. They live in different social structures,
ranging from pair-bonded to large groups of up to 100 individuals.
The distribution of these monkeys covers a vast area, from southern Mexico to
northern Argentina. They inhabit various ecosystems, including tropical
rainforests, subtropical rainforests, savannah-like areas, and even mountainous
regions.
Over millions of years, unique animals evolved there, like marsupials, sloths,
and large carnivores. Then, some animals arrived by chance events - rodents
from Africa around 40 million years ago and primates around 36 million years
ago.
Ancestors of these monkeys along with rodents ended up in South America around
the period called the late Eocene. When new world monkey ancestors appeared in
South America, during the Late Eocene, Africa and South America were separated
by a long ocean stretch. How was it possible for these animals to make it to
South America? Different possibilities have been proposed.
The idea that New World monkeys (Platyrrhini) originated from primitive
prosimians in the ancient supercontinent Gondwana seems unlikely. The
separation of Africa and South America happened much earlier than the estimated
divergence time for these monkeys. The proposed route through Europe and North
America, which were still connected around 26 million years ago, is also
improbable as North America and South America were still separated until 5
million years ago. Additionally, the absence of primate fossils in Antarctica
challenges this notion.
New World monkeys, or their ancestors, may have reached South America from
Africa around 36 to 40 million years ago on natural rafts made of floating
vegetation or debris. This "waif dispersal" likely occurred during
the Eocene epoch when continents were differently positioned, and ocean currents
facilitated unintentional transport across the Atlantic Ocean. Once these rafts
reached South America, the monkeys adapted to the new environment, eventually
leading to the evolution of the diverse group of New World monkeys we have
today. This natural rafting event is a proposed explanation for the presence of
monkeys in South America.
While it might sound like a fantastical journey, this type of natural rafting
event has been proposed as a way for organisms to colonize new islands and
continents
During a warm period around 40.5 million years ago, there were intense weather
events, especially floods in tropical areas. This might have increased the
chances of riverbanks breaking up, creating more opportunities for these
animals to be carried away on natural rafts.
During that time known, Africa and South America were significantly closer. The
span of the Atlantic Ocean between the two continents measured about 930 to
1,300 miles apart compared to the modern expanse of 1,770 miles. In addition,
the buildup of glaciers in Antarctica around that time caused sea levels to
drop, making the passage shorter than it is today.
Similarly, floods might have also helped animals travel between South Asia and
Africa. In simpler terms, favourable environmental conditions and shorter
distances might have played a role in these ancient animals travelling across
oceans on natural rafts.
Monkeys thrived in South America, partly because they didn't have strong
competitors for their specific lifestyle. Early monkeys were tree-dwellers,
eating fruits and insects. Unlike other mammals in the area, they were diurnal
and social, which likely gave them an edge. This success led to various
adaptations, like swinging through trees called semi-brachiation, specializing
in certain fruits, eating gum, being omnivores, and even using tools.
The Holocene extinction around 10000 years ago, where 50 large-bodied South
American genera were lost, led to competitive release and the expansion of
primate abundance. However, it's tough to reconstruct these details because the
tropical rainforest, where these events occurred, isn't great for preserving
fossils due to the hot, humid climate and thin soil layer.
New findings in the Peruvian Amazon indicate that not just one, but multiple
groups of monkeys from Africa made it to South America. Along with the African
ancestors of rodents, at least three different groups of early monkeys from
Africa reached South America around 32-36 million years ago.
These distinct groups, discovered so far, have been named Ashaninkacebus,
Perupithecus, and Ucayalipithecus, related to three distinct groups of Old
World primates. Ashaninkacebus appears related to Eosimiidae, Perupithecus to
Oligopithecidae, and Ucayalipithecus to Parapithecidae.
What's interesting is that Ashaninkacebus simpsoni is not related to the
African monkeys but has connections to monkeys from South Asia.
The eosimiids are mainly found in South Asia. Interestingly, a fossil from
Tunisia in Africa, named Amamria, is similar to these Asian monkeys. If Amamria
is indeed one of these eosimiids, it suggests that these Asian monkeys somehow
travelled to Africa during the middle Eocene.
What's even more surprising is the idea that these Asian monkeys might have not
only reached Africa but continued their journey across the Atlantic Ocean to
South America. It's like they made a stopover in Africa before moving on to
South America. So, the route could have been from South Asia to Africa and then
across the Atlantic, with Africa acting as a kind of rest stop for these
ancient monkeys and rodents travelling across continents.
Genomic analyses suggest that the divergence between New World monkeys and
Catarrhini or Old World monkeys and apes happened around 43.5 million years
ago. Platyrrhines share a common ancestor from 19 to 24 million years ago.
The specific monkeys, Ashaninkacebus and Perupithecus, were quite different
from the monkeys that came later to South America. Now, the big challenge is to
figure out exactly how these early South American monkeys are related to the
first-known monkeys in the region, called "true" platyrrhines. To do
this, we need more fossil evidence, and scientists have to explore more in
tropical areas where we haven't looked much yet. It's like solving a big puzzle
that will help us understand the early history of these monkeys in South
America.