Continents' constant shifting is one of the first things you learn when you study the geologic history of Earth. South America fits into Africa like a puzzle piece, after all. Back 200 million years ago, everything was combined in a supercontinent called Pangea.
Rocks alone seem to show that the breakup happened 180 million years ago. But a team of Australian scientists think that you should be able to see the split and continuing shifts written into the history of how animals have evolved. So that's what the researchers did, and they accomplished this by analysing a large group of species' evolution and compared them to the date of the breakup of Pangea.
Pangea first split into the continents Laurasia and Gondwana around 180 million years ago. The split led to a continuing shuffle and further splits until the landmasses assumed their present position. These splits led to species being isolated on either side. Once isolated, these species began their separate evolutionary journeys.
But there's a controversy in this field of biogeography, noted Liliana Katinas, a professor at the National University of La Plata in Argentina who was not involved in the study, in an email to Gizmodo. Do new species come from animals populating new territory (called dispersal), or did populations get separated during Earth's breakup (called vicariance)? New methods in biogeography put many in favour of dispersal as the prime factor. "The authors of this paper are trying to return to the previous ideas and re-emphasise the role of the rupture of continent in some organisms' distribution," said Katinas.
The researchers used the timetree.org database to find data on molecular clock divergence dates and to see how species diverged based on their genetics and proteins in their bodies. They admitted that the database varies in its quality, but it's just about the most comprehensive data available. They picked pairs of freshwater and terrestrial vertebrate groups that were descended from common evolutionary ancestors and represented on at least two continents, and looked at their divergence times.
After lots of number crunching and analysis, the divergence dates of the major groups separated by continents seemed to line up well with the continents separating geographically, according to the paper published this week in the Proceedings of the Royal Society B. This adds further evidence to the importance of vicariance, over dispersal, as the cause for further speciation.
The study impressed at least one other researcher. "Their paper is valuable in showing an expected pattern of organismal divergence is correlated across deep time," Joe Cracraft, Chair of the Division of Vertebrate Zoology at the American Museum of Natural History, told Gizmodo in an email. "Other studies have tried this, but not with as much success."
There's room for scepticism, though, that the authors Katinas and Cracraft both point out. Katinas immediately pointed to the timetree.org data as a potential source of error. Also, things such as land bridges and uncertainties in what landforms actually looked like might cloud the accuracy of the data, and there are plenty of moving pieces the model had to correct for. And "another caveat is that estimating time trees is not straightforward," said Cracraft, "but their statistical approach helps account for some of that."
But ultimately, this paper is swinging the pendulum between two competing ideas, said Katinas, gradually reaching a balance between them.