How did ancient monkeys arrive in the New World?

02/23/2014 12:00 AM

02/23/2014 5:08 PM

This isn’t a story about whether humans evolved from monkeys. But we share many of those primates’ outward features and even organizational systems in the brain, so they’re a perpetual scientific curiosity – especially in terms of how they evolved.

Richard Kay, an evolutionary anthropologist at Duke University, recently finished an in-depth analysis of monkeys’ evolution and migration to the Americas tens of millions of years ago. His findings shed new light on the scope of their geographic footprint while underscoring the vital role of the Amazon Basin in their expansion and interrupted lineage.

Researchers estimate that monkeys arrived in South America 37 million to 40 million years ago – having diverged from apes and Old World monkeys – but the first fossil evidence of them in the Western Hemisphere is dated 11 million years later. The Amazon’s excessively humid and wet climate is the culprit.

“A key to all of this is the great importance of the Amazon Basin,” Kay said. Though it’s a haven for primate biodiversity, “it’s hard to collect fossils in an area of such heavy rainfall and rainforests. Mostly what we’ve been able to do over many years is to sort of pick around the edges and try to read the story of what was happening in the central core of South America, for which we have virtually no evidence.

“The challenge is, how do you go about interpreting what’s going on in one region based on the wastes and jetsam of animals that find their way out of the tropics at certain times, when climates were more equitable?”

Kay, who has collected fossils of monkeys in South America since 1981, has worked with others in peripheral Amazon areas such as Chile and Argentina. By studying fossils from there and piecing together data on geology, ancient climates and evolutionary ties, he uncovered several patterns in monkeys’ evolution and migration in the New World. A key determination was that once in the Americas, monkeys evolved new forms and spread as far north as the Caribbean and as far south as Patagonia.

Even areas near the Amazon Basin ultimately felt climate and geological impacts, he said. The Patagonian climate eventually turned dry and cold, causing monkeys’ extinction. And rising sea levels and the arrival of humans killed off monkeys in the Caribbean during the past 6,000 years.

New World, old mystery

Today there are about 150 species of New World monkeys (also called Platyrrhini, which means “flat nose”) living in Central and South America. They include capuchins, marmosets and tamarins and are divided into categories that include squirrel monkeys, night monkeys, owl monkeys, howler monkeys, spider monkeys, woolly monkeys and woolly spider monkeys.

New World monkeys are mostly distinguished from their Old World counterparts – found in tropical Africa and Asia – by their flat noses, small to medium size, and tree-dwelling tendencies.

How monkeys reached the New World remains a mystery. “We don’t know how they got to South America, just that they got here,” Kay said.

Some scientists theorize that these monkeys’ ancestors migrated from Africa by crossing the Atlantic on a vegetation raft or crossing a temporary land bridge that later submerged and isolated them. Once in the Americas, they kept evolving separately from the lineage that produced today’s Old World monkeys and apes.

“You can really think of South America as the continental drift, having detached itself from Africa 90 to 120 million years ago,” Kay said. “The animals that were living in South America at the time those continents diverged were pretty much on their own and evolved into a variety of different kinds of animals in isolation.”

How he did the study

These and other conclusions ideally result from molecular evidence gathered by sequencing the DNA of living monkeys, but genetic material sometimes deteriorates. So in cases of extinct species, researchers study even the most minute differences in the fossils’ size, structure and shape.

Kay’s most recent study, which took more than a year, entailed the same kind of labor-intensive methodologies – with a high-tech twist.

He examined 399 different features of teeth, skulls and skeletons from 16 living and 20 extinct monkey species from South America and Africa. Then he built a family tree by using software that reconstructs evolutionary relationships. He compared that to a tree built strictly from the molecular studies of living species in order to determine whether the two types of studies showed similar or different findings. Most were similar.

“The first thing you’ve got to do is come up with the characteristics you’re going to look at … find features that are pretty consistent within the species and distinguish these species from another,” Kay said.

“How do they look different? If two animals looked a little different, I would write that down as a possible characteristic. Then, within the species, I would look for variations. … So you winnow out a lot of things. And then, of course, there are many people publishing things, so you have to stay on top of any new findings.

“Once you have this data and have evaluated it, you have to perform analyses that are fairly complex, but there’s very good computer software to do it. There are computer algorithms that are co-opted to examine similarities among different organisms.”

He cautioned that few conclusions are infallible: “Anytime you come up with a reconstruction, you’ve got to be aware that it’s probably right in most ways but probably also wrong in some others. Science is, by its nature, something that you make a first stab at it and then you make a further correction.”

Why it affects us

Given that climate change and geological phenomena are constants in the monkeys’ history, it was suggested to Kay that these components’ enduring effects on various ecosystems and living things is the most important takeaway of his research.

“You hit the nail on the head, as far as I’m concerned,” he said. “You can make some sense of these immigration patterns when you can understand the underlying geological factors and climate changes. … And monkeys can be great indicators of climate change.”

In fact, he said, building more reliable climate change models would be instrumental in this and other research. “If the models are good, we ought to be able to predict backward just as easily as forward.”

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