Two recent papers use ancient DNA to show admixture between mammalian species during the ice ages.
Physicists are often discussing the possibility of time travel, venturing back into the distant past to relive history firsthand. Sounds very exciting, but until the construction of time machines, we will need to rely on other means to explore the past. Biologists, for example, use fossils. And nowadays, it is even possible to extract and sequence DNA from fossil remains. Two recent papers have applied this technique to uncover the evolutionary history of elephants and bears.
Let’s start with the elephants. Today, three species of these majestic giants roam the Earth. The forest elephant (Loxodonta cyclotis) and the savanna elephant (Loxodonta africana), and the Asian elephant (Elephas maximus) in – you guessed it – Asia. I had the honor of observing these animals from close range during a trip to Tanzania a couple of years ago (see photo).
An elephant crossing the road in Tanzania
These three elephant species are the only survivors of a much larger order (Proboscidea). In their paper, Eleftheria Palkopoulou and her colleagues focus on two extinct groups within this order: the mammoths and the straight-tusked elephants. They sequenced the genomes of two mammoth species – woolly mammoth (Mammuthus primigenius) and Columbian mammoth (M. columbi) – and one member of the straight-tusked elephants (Palaeoloxodon antiquus).
Analyses of these ancient genomes revealed hybridization between several species. The straight-tusked elephants seemed to have received genetic material from at least three sources: the ancestors of the forest and savanna elephants, woolly mammoths and forest elephants. And in North America woolly mammoths and Columbian mammoths were interbreeding. The family tree of the elephants is getting quite bushy.
Artist impression of a straight-tusked elephant (from: http://www.museumoflondonprints.com)
Several studies have shown that brown bears (Ursus arctos) and polar bears (U. maritimus) have hybridized in the past. But what happened exactly? Currently, scientists are discussing two main scenarios. In one model – the population conversion model – the warming climate at the end of the last ice age (about 11,000 years ago) allowed brown bears to disperse into the range of polar bears and interbreed with them. The alternative model states that hybridization occurred before the last ice age and did not continue afterwards.
To discriminate between these scenarios James Cahill and his colleagues sequenced DNA from ten cave-preserved bones from Ireland. The age of these fossils ranges from about 40,000 to 4,000 years ago. The analyses revealed that genetic ancestry from polar bears in the brown bear genomes peaked at the end of the last ice ages and declined until the extinction of the Irish population. This pattern is consistent with the population conversion model, but it does not rule out the possibility of hybridization before the end of the last ice age. Sequencing older fossils might be necessary to confidently support one of the models.
A second-generation polar-grizzly hybrid on display in the Ulukhaktok Community Hall, Ulukhaktok, Canada (from: http://sciencenordic.com/)
The importance of ice ages
Regardless of which model is more accurate, hybridization between brown bears and polar bears seems to be related to the ice ages. Similarly, hybridization events with straight-tusked elephants were dated to ~120,000 years ago, which overlaps with the glacial cycles. In fact, my own research (on the evolution of geese) also highlights the pivotal role of the ice ages in hybridization:
The reconstruction of historical effective population sizes indicates that most species showed a steady increase during the Pliocene and Pleistocene followed by population subdivision during the Last Glacial Maximum about 110,000 to 12,000 years ago. The combination of large effective population sizes and occasional range shifts might have facilitated contact between diverging goose species, resulting in the establishment of numerous hybrid zones and consequent gene flow.
Hybridization as the norm
These complicated histories show that we need to take hybridization into account when studying the evolution of mammals. Indeed, in the elephant study the authors write that “our results […] thus add to the growing weight of evidence in favor of the view that capacity for hybridization is the norm rather than the exception in many mammalian species over a time scale of millions of years.”
Cahill, J.A. et al. (2018) Genomic evidence of widespread admixture from polar bears into brown bears during the last ice age. Molecular Biology and Evolution. https://doi.org/10.1093/molbev/msy018
Ottenburghs, J., Megens, H.-J., Kraus, R.H.S., van Hooft, P., van Wieren, S.E., Crooijmans, R.P.M.A., Ydenberg, R.C., Groenen, M.A.M. & Prins, H.H.T. (2017). A History of Hybrids? Genomic Patterns of Introgression in the True Geese. BMC Evolutionary Biology. 17:201 https://doi.org/10.1186/s12862-017-1048-2
Palkopoulou, E. et al. (2018) A comprehensive genomic history of extinct and living elephants. PNAS. https://doi.org/10.1073/pnas.1720554115