Monday 24 October 2016

Understanding the ancestry of the European Bison.

The European Bison, or Wisent (Bison bonasus), is one of a very small number of megafaunal (large) animal species to have survived from the Pleistocene in northern temperate latitudes. However the origin of the European Bison is less clear than it would immediately seem, as the earliest known fossil specimens date only from the Early Holocene (i.e. less than 11 700 years ago), prior to which Europe was inhabited by a different Bison species, the Steppe Bison (Bison priscus), which ranged from Britain in the east across Europe, Asia and Beringia (the landmass between Alaska and the Russian Far East which was exposed during the Pleistocene glaciations when the sea level was lower) and into western Canada. The Steppe Bison is considered to be the ancestor of the American Bison (Bison bison), but its relationship to the modern European Bison is less clear, as the European Bison, while clearly related to the American Bison, is genetically more closely related to Cattle (Bos spp.). This could potentially be due to recent hybridization between the species, the European Bison having gone through a number of population bottlenecks as it came into conflict with Humans and domestic animals, with the population having fallen to just twelve individuals in the 1920s, but hybrids between Bison and Cattle are usually infertile, making this a somewhat doubtful hypothesis.

In a paper published in the journal Nature Communications on 18 October 2016, a team of scientists led by Julien Soubrier of the Australian Centre for Ancient DNA at the University of Adelaide describe the results of a new study of the ancestry of the European Bison, which takes into account both archaeological and genetic evidence.

Soubrier et al. sequenced DNA from 64 Bison specimens from Late Pleistocene sites (from before 50 000 years ago to about 14 000 years ago) across Europe. 38 of these specimens, from the Caucasus, Urals, North Sea, France and Italy, were found to belong to a previously unknown Bison lineage, identified as Clade X (in biology a clade is a group of organisms with a common ancestor; a clade includes all the organisms descended from that ancestor and no organisms not descended from that ancestor). Based upon the relationship between Clade X and the European Bison, the two species shared a last common ancestor about 120 000 years ago (during the Eemian Interglacial), however like the European Bison, including Ancient European Bison used in this study (and unlike the American Bison) members of Clade X appear to derive about 10% of their DNA from a Cattle (Bos sp.) ancestor.

This discovery is interesting as it rules out the possibility of recent hybridization between Modern Cattle and European Bison, instead suggesting that the species came about as the result of a hybridization between ancient Steppe Bison and the Aurochs (Bos primigenius), a large Pleistocene animal thought to have been ancestral to modern Domestic Cattle. Further examination of the Bison samples in the study revealed that they all derived their mitochodrial DNA (mtDNA) from a Bos ancestor, but none of them any Y-chromosome DNA, This implies that both the European Bison and Clade X came about as a result of a hybridization between male Steppe Bison and female Aurochs, not implausible given that in all known Bovid species males tend to capture harems of females which they defend aggressively against other males, and that while the Aurochs were big animals, Steppe Bison were even larger. However whether or not the European Bison and Clade X arose from a single hybridization event, a cluster of such events or completely unrelated events could not be determined.

Soubrier et al. further observe that two distinct morphologies of Bison are recorded in European Pleistocene cave art. A total of 820 depictions of Bison are known from European cave art (21 % of all known animal depictions from the Pleistocene of Europe), and all of these fall into two morphotypes (shapes). The first is a long-horned animal with robust forequaters and a distinct hump, that appears in art dating from before the Last Glacial Maximum, roughly between 22 000 and 18 000 years ago. The second form is a more slender animal with small, recurved, horns and a small hump, which appears in Magdalenian art, roughly between 17 000 and 12 000 years ago. These differences have previously been regarded the expression of different artistic styles, however Soubrier et al. reject this hypothesis. Two different Bison morphologies (a robust form and a slender form) have also been noted from bones dredged from beneath the North Sea).

Cave painting example of steppe Bison-like and Wisent-like morphs. (a) Reproduction from Lascaux cave (France), from the Solutrean or early Magdalenian period (about 20 000 years ago). (b) Reproduction from the Pergouset cave (France), from the Magdalenian period (less than 17 000 years ago). Soubrier et al. (2016).

The oldest specimens with a European Bison genotype dated from before 55 000 years ago, while the oldest Clade X specimen dated from 23 000 years ago, and the youngest Steppe Bison from 19 000 years ago. However all known specimens from between 50 000 and 34 000 years ago are Steppe Bison. Environmental reconstructions and stable isotope studies of remains (which can reconstruct the environments in which ancient organisms live by studying the ratios of different carbon and nitrogen isotopes in their bones) suggest that the European Bison and Steppe Bison inhabited different environments, with the Steppe Bison dwelling in cold, tundra grasslands and the European Bison favouring a warmer, more mixed environment.

Based upon this Soubrier et al. suggest that there were two distinct Bison species present in Europe for much of the Late Pleistocene. The Steppe Bison favoured cold grassland environments and expanded its range during colder periods, disappearing at the end of the Pleistocene, probably due to a mixture of a loss of much of this environment and a rising Human population armed with better hunting tools. The European Bison arose from one or more hybridization events between the Steppe Bison and the Aurochs, but was able to persist as a ecologically separate entity due to a preference for a different environment, favouring warmer landscapes with mixed vegetation. This species has also been severely pressured by expanding Human populations, but was better able to cope with the changing climate and has persisted till modern times.

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