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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