Credit: Alistair Evans, David Jones and IMPPS
Reading aloud Between a mouse-sized mammal and an elephant, there are at least 24 million generations of evolution. This is the result of the calculations of an international research team, which has evaluated the course of the size development in the pedigree of some species. On the other hand, the counter-directed development is obviously much less complicated, the evaluations revealed: The evolution to dwarf forms of originally large animal species took place 10 times faster than vice versa. From the gigantic blue whale to the tiny dormouse? All mammal species living today have a common ancestor, on which their family tree can be traced. After the extinction of the dinosaurs about 65 million years ago, the primitive mammals fanned into many species, which sometimes differ drastically in their body dimensions. Against this background, the researchers have now pursued the question of how long the evolution needed for these size changes, for example, to transform a mouse-sized ancestor in the pedigree of elephants into the largest present-day land creature. The team led by Alistair Evans of the Australian Monash University School of Biological Sciences has evaluated data on the evolution of 28 mammalian groups over the last 70 million years documented by fossil finds. Among them, for example, the pedigrees of the trunks, primates and whales.

As a measure of the development speed, the researchers chose the number of generation sequences needed, because only in this case, new gene variants that allow development to more size arise. This is the deciding factor in evolution, not time, explain Alistair Evans and his colleagues. The generation sequence as a measure also allowed a meaningful comparison between life forms with different lifetimes. The model researchers finally came to the maximum development speed of the property body mass: For an increase in size, which corresponds to that between a mouse and an elephant, the evolution of land mammals therefore requires at least 24 million generations. If, on the other hand, a certain size had already been reached, it was obviously faster: An already rabbit-sized animal species required about ten million more generations before a species with elephantine dimensions could emerge.

Whales became giants comparatively fast

An exception to the rule revealed in the study of the evolution of the whales: The evolution of small marine mammals to the giants of the oceans was carried out twice as fast as the studies in the land mammals. The researchers suspect that increasing the size of the water, because of the load-bearing properties of the medium, requires less costly adjustments than on land. To the "model"? To evolve an elephant, evolution must master severe static challenges, believe Alistair Evans and his colleagues. display

The researchers also examined the speed of the opposite development in their investigations? How many generations are needed to develop a smaller one from a large animal species? An example of this are today's dwarf forms of the hippo or the elephants and mammoths that once existed on some islands. Here, fossils from the genealogical tree of these species document that the shrinkage was comparatively fast: The development of the pygmy elephants, for example, took ten times less generations than the reverse evolution of a sheep-large ancestor of elephants to the large species.

Alistair Evans (Monash University School of Biological Sciences, Australia) et al .: PNAS, doi: 10.1073 / pnas.1120774109 © Martin Vieweg


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