Hopping to Wyoming

As I browsed the new papers published this week in the Proceedings of the National Academy of Sciences, I was brought up short by this frolicsome picture. It's a nice example of the visual display of information done right. It shows the spread of primates 55 million years ago across the Northern Hemisphere. I'm always game to learn about what primates were up to in Wyoming and Greenland. But this picture--and the paper that goes with it--have an extra value. They offer some clues to what sort of world we may be creating by pumping billions of tons of carbon dioxide into the atmosphere.

Paleontologists have known for a long time that something remarkable happened around the time the Paleocene period ended and the Eocene began, about 55 million years ago. The oldest fossils of many groups of mammals appear for the first time around then, including hoofed mammals, whales, bats, and primates. Mammals also made tracks, spreading to continents where, at least according to the fossil record, they had not lived before. At the same time, the world saw a rise in extinction rates, particularly among some single-celled deep sea organisms.

Geologists have also found much to ponder from this moment in history. Rocks from the end of the Paleocene contain peculiar levels of isotopes of carbon and other elements, pointing to a dramatic change in the planet's global chemistry. Scientists now estimate that somewhere between 1500 and 4500 billion metric tonnes of carbon was released into the atmosphere and oceans over several thousand years. Scientists are debating exactly where all that carbon came from. Some argue that buried methane deposits came belching out of the sea floor. Other possibilities include a sudden rise in fires.

However this carbon turned up, it changed the world. The atmosphere warmed by an estimated five to nine degrees C. Formerly dry regions turned muggy. The oceans warmed as well, and its circulation shifted. As carbon dioxide dissolved into the oceans, they turned acidic. After the carbon was initially released 55 million years ago, the planet continued to warm, acidify, and otherwise change for the next 75,000 to 90,000 years. It then took another 70,000 years to recover to something akin to its previous state. (A quick, free, up-to-date summary of the evidence was published this year in the journal Eos: pdf.)

In recent years, paleontologists and geologists have been getting together to pool their results, with some striking results. Geologists have put precise dates on the shifts in carbon levels and other chemical changes at the end of the Paleocene. Paleontologists can identify those shifts in the rocks in which they find fossils. This knowledge lets them narrow the ages of some fossils down to intervals of a few thousand years. History makes more sense with better resolution. To say that Napoleon and Charlemagne lived within 1000 years of each other isn't saying much at all.

In the paper from which this figure comes, a team of paleontologists looked at the timing of primate evolution around the heat spike. Until now it hasn't been clear exactly when primates first expanded their territories, nor has it been clear which path they took--where they originated, and where they moved to. They focused their attention to six species belonging to one genus, Teilhardina. Teilhardina is the oldest primate from Europe and Asia, and it is tied for oldest with another primate in North America. It is also the oldest "true" primate (euprimate), sharing many of the features that set living primates off from extinct relatives and ancestors. Its fossils indicate that it was a small tarsier-like primate that searched for food during the day, living exclusively in trees.

The scientists found that fossils of Teilhardina from Asia are the oldest, followed by fossils in Europe, finally followed by fossils in North America. A nice bit of congruence emerged when they drew an evolutionary tree of the Teilhardina species along with a couple other early primates. The paleontologists found that the Asian species had the deepest branch, and that the European and North American species shared a more recent common ancestor.

The new dating on the fossils indicates that this spread from Asia to Europe and North America took less than 25,000 years, right as the carbon levels were soaring. One must always take these results with a grain of salt, because paleontologists can never say for sure that they've found the oldest fossil of any species. However, they can be relatively confident when the rocks from the age they're interested in are loaded with fossils. And that just so happens to be the case for primates 55 million years ago.

As this picture suggests, it now appears that primates (or at least primates like Teilhardina) originated in Asia and moved to Europe. They then moved from Europe into Greenland and then into North America. The paleontologists suggest that a number of mammals may have taken the same path to North America (rather than going over the Bering Land bridge, as has been suggested in the past). Ocean levels dropped around 55 million years ago, and so the primates and other mammals may have been able to move over land bridges between the continents. Teilhardina's particular way of life also implies that this journey had to have passed through a continuous belt of lush forests. Greenland would have had to have been very green indeed for primates to pass through it. The warm, humid climate would have suited such forests well. It just so happens that paleobotanists have been looking at what happened to plants 55 million years ago, and they've documented a swift spread of tropical trees northwards through North America over 10,000 years. So it appears that global warming laid down the path for the primates, and they raced along it.

The journey of Teilhardina is one we would do well to consider. We are now injecting carbon dioxide into the atmosphere at about the same rate at which it was released 55 million years ago. The Paleocene-Eocene boundary may offer some clues to how the world's ecosystem may respond. Some species may become extinct--particularly marine species that won't be able to cope with the acidity of the oceans. Others will spread. Teilhardina covered 20,000 kilometers in less than 25,000 years. That's hardly supersonic. In fact, it matches the rate of dispersal scientists observe in mammals today--about a kilometer a year. But Teilhardina spread at this rate year in and year out, and before too long (geologically speaking) it had circled the globe. If we ever do figure out a way to cut off our carbon emissions, the carbon we've already put in the atmosphere may last for a long time. In the Eocene, the planet took 70,000 years to recover. But the ecological changes were far more durable. Primates managed to stick around in North America for about 20 million years, until the climate had cooled too far to support the forests they depended on. The Earth's planet has continued to cool and carbon dioxide levels have continued to drop. That means that the current climate change will not be a perfect replay of the crisis 55 million years ago. But it does serve as an example of how a pulse of global warming can do some remarkable things, such as putting primates in Wyoming for 20 million years.