Monkey predecessors so tiny that one might loll in a tablespoon migrated across a land bridge connecting Siberia to North America more than 55.8 million years ago, and eventually colonized much of what was then a lush, subtropical continent, according to research published today.
Fossils uncovered in Mississippi reveal the deepest and earliest penetration of primates into the New World, according to the research by Pittsburgh's Carnegie Museum of Natural History, and might cause paleontologists to revise their views on how these ancient ancestors of modern-day lemurs, tarsiers, monkeys, apes, and humans reached America and Europe.
The journey, which spanned millennia and countless generations of the sap-slurping, tree-dwelling creatures, occurred against the backdrop of dramatic climate change "comparable in terms of both rate and magnitude to the current phase of global warming," said K. Christopher Beard, paleontologist at the museum and author of the paper in the journal Proceedings of the National Academy of Sciences.
"For these creatures to have made it, there needs to have been an unbroken subtropical forest running from southeast Asia up through Siberia and then down the [Pacific] coast," he said in an interview.
The previously unknown and long-extinct mammal was named Teilhardina magnoliana, after the Magnolia State. The Mississippi fossils are fragmentary and the dating is all but certain to spark hot debate among primate paleontologists.
"Beard has a history of offering provocative papers ... this should generate a number of interesting responses," said Herbert H. Covert, professor of anthropology and specialist on primate evolution at the University of Colorado. But he described the Carnegie research as exciting and important.
The dating is critical. If the primate remnants are truly 55.8 million years old, that means the species was well established in North America shortly before sinking sea levels of that era may have opened a narrow land passage between Greenland (then attached to the continent) and Scotland.
That link has long been theorized to be the route that early primates followed from the Old World to the New -- but Teilhardina magnoliana, according to the new research, was already frolicking among the beach fronds on what was then the coast of the Gulf of Mexico. The fossils were found in sands formed by long-vanished tidal channels near present-day Meridian, Miss., now some 140 miles inland.
Moreover, since the research indicates the Mississippi fossils are older and more primitive than related Tielhardina fossils found in Belgium and France, it raises the possibility that Europe's first primates arrived by way of North America, not directly from Asia.
"They seem to have taken the long way around," Beard said. "It's very counter-intuitive."
Teilhardina is estimated to have weighed about one ounce and was an "acrobatic leaper and proficient climber" that survived on a diet of "insects, fruits, sap, and [tree] gum," according to a description by the Carnegie Museum. In appearance, it was closer to the saucer-eyed tarsiers today inhabiting the Malaysian archipelago than to modern monkeys or apes.
Fossils of a closely-related species have also been found in Wyoming's Bighorn Basin, but the Mississippi creatures appear both older and somewhat less-evolved than their Rocky Mountain cousins -- suggesting they were earlier North American colonizers.
Primitive primates inhabited North America for millions of years before disappearing in the face of a global cool-down starting about 34 million years ago. Today, the only living primates "native" to North America are homo sapiens. There are robust monkey populations in Latin America, but they originated in Africa, not Asia, and seem to have arrived on "floating islands" of jungle debris -- not by land routes -- long after northern primates had become extinct.
Not all scientists buy Beard's theory that primates reached North America solely by way of a Siberian-Alaska link, then skittered on to Europe. They note that his claim for T. magnoliana's precedence involves only tens of thousands of years -- just the barest sliver of geological time -- and depends on a dating technique called sequence straigraphy. This involves using coastal features to compare the dates of fossils found across vast spans of territory because sea level would remain constant around the globe.
"There is ambiguity over which of these occurrences may be the oldest," said Kenneth D. Rose, professor of anatomy and evolution at Johns Hopkins University School of Medicine, and a specialist in extinct primates. "Even if Beard is correct, it does not prove all [primate] dispersal was from Asia to North America to Europe. In fact, dispersal could have been in both directions. ... The debate is far from settled."
Earlier research, in which Rose participated, suggested that the rapid spread of Teilhardina through the world was driven by the arrival of the Paleocene-Eocene Thermal Maximum, a dramatic global warming period caused by a rise in naturally-occurring greenhouse gases. The nearly-simultaneous appearance of Teilhardina species in Asia, Europe, and North America closely correlated with climate change, which allowed forests -- serving as roadways for restless tree dwellers -- to flourish far north of their present boundaries.
Since there were no polar ice caps to melt at the time, the Paleocene-Eocene is associated with drops in ocean levels (not the rising seas forecast for the 21st century's climate shifts) caused by continental drift that altered the volume of ocean basins, according to scientists.
Teilhardina remains an enigmatic genus. Scientists generally agree that it was a true primate -- or euprimate -- but are a little hazy as to what kind. In Beard's view, Teilhardina may have occupied a niche somewhere between the so-called lower primates -- animals that evolved into today's lemurs, bush babies, and tarsiers -- and higher primates, such as apes, monkeys, and humans.
"They are critical animals, evolutionarily," he said. "It has been difficult to reconstruct how these tiny creatures spread over so much of the globe during a period of rapid climate change and sea level fluctuation. These earliest primates came during an incredibly dynamic time in earth history."