Biogeography: Animal History

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Biogeography: Animal History
Biogeography: Animal History

Animal Story

We can only guess what happened millions of years ago - after all, nobody was there. Fortunately, our inanimate environment provides a we alth of clues that can be used to reconstruct events. And sometimes biology turns it into a really well-rounded story.


A few million years or not are mostly negligible from a geologist's point of view. But sometimes researchers would like to know more precisely when which continents split up or came together, how quickly many mountains rose or sea basins closed. And so they try to uncover the history of the earth in as much detail as possible using classic tools such as fossils or isotope analyses.

Biogeographers are also keenly interested in these geotectonic foundations - in their profession it is important to reconstruct the evolution and spread of animals and plants over the course of millions of years. Geographical barriers such as mountains, sea basins or continents drifting apart play a decisive role, as they usually cause a split in the family tree of the organisms under consideration. The geologists' data therefore help them to substantiate their kinship analyses.

Or vice versa: Researchers led by David Wake from the University of California at Berkeley chose the Asian salamander family Hynobiidae to date geotectonic events in their homeland. On the basis of genetic analyzes of the mitochondrial DNA of today's representatives, they determined molecular clocks for the evolutionary development of the group - a not entirely uncontroversial method that sounds easier than it is. For example, when comparing other such data from the amphibian world, Wake and his colleagues found that they could not assume a uniform molecular clock for the entire family. The members of different family groups thus developed at different speeds – a considerable source of error from the point of view of critics.

According to the researchers' calculations, the last common ancestor of all angle-toothed newts that still exist today lived around 110 million years ago. They probably inhabited flowing waters and not, as is otherwise assumed, ponds, because this way of life maintains the original forms developed by Wakes and co. As a result of the reorganization, however, pond dwellers are now in different groups - but this is not a problem: it only means that various adaptations to life in still waters have developed independently of one another and were not inherited from a common ancestor. Not an isolated case in evolution.

The scientists see the place of origin in what is now northern China. Why? Because that is where most of the fossils have come from so far – and because one of the genera that split off at the earliest occurs there. In addition, researchers have so far known remains of the family from Europe, but not from North America. If the angle-toothed newts had emerged in Europe at that time, they would certainly have settled in America via the land connection between the northern continents that still existed at the time, Wake and Co explain. However, they first had to make their way from East Asia - which they then had to do with the lack of the Himalayas was still open in a flat landscape with a humid climate. By the time they finally reached the western edge of Eurasia, about 50 million years ago, North America had already vanished, preventing them from making the leap to the New World.

Without the corresponding environmental pressure in an environment that was pleasant for them, not much had happened in terms of evolution in these millions of years: The scientists only discovered the first branches in the young family tree at the beginning of the Tertiary 65 million years ago, which then accumulated in the Eocene. Barriers must have arisen to explain this pattern. They are found quickly: India had completed its ocean crossing at that time and had crashed into the Eurasian continent. The newly docked landmass to the south severely altered the precipitation patterns in the hinterland. The drying up of Inner Mongolia, which must have started around 50 million years ago, made the once extensive homeland extremely uncomfortable for the amphibians, who needed moisture and were not very keen to migrate, and which were now isolated by dry regions.

Just ten million years later, researchers observed a similar fragmentation in their analyses. Among other things, a genus now appears for the first time whose members are the only ones of the Hynobiidae that still exist above 2500 meters. Accordingly, at least the eastern part of the Tibetan Plateau must have exceeded this level by 40 million years ago, Wake and his colleagues conclude - another sign of the much-discussed rapid uplift of the plateau. The last coup in speciation followed about 24 million years ago. At that time, the scientists conclude, Southeast Tibet witnessed intense mountain building.

The question remains whether the researchers' analyzes will be well received. Because the maxim often applies: if you look alike, you're probably related. In the case of the angle-toothed newts, for example, there are species with four and those with five toes on the hind feet. So it makes sense to assign the four toes to one group and the five toes to another, especially if other characteristics also match. But sometimes, and that's the way science works, simple assumptions are simply wrong. Or at least worth discussing. So it would be in the case of the Hynobiidae: According to Wake's family tree, morphologically similar groups would be torn apart and completely variegatedly reassembled. There should therefore be far more discussion material here than in the molecular-biological dating of geotectonic events - because a few toes more or not are certainly more controversial than a few million years.

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