Ancient DNA retrieved from different mammoth species is illuminating a complex evolutionary picture.
Picture: Beth Zaiken/Centre for Palaeogenetics
In a paper published in Nature a few days ago, a team of palaeontologists led by Tom van der Valk of the Centre for Palaeogenetics, Stockholm, Sweden, report recovering useable DNA from the million-year-old remains of mammoths. This sets a new record, the previous one being established in 2013 when a team from the University of Copenhagen led by Ludovic Orlando reported extracting DNA from the leg-bone of a horse dated to 560-780 thousand years old.
This mammoth DNA was extracted from three teeth, two of which have been dated to over a million years old, which had been preserved in Siberian permafrost. According to this report in Nature:
The authors dated the mammoths using biostratigraphy, in which faunal remains at the sites where the molars were collected are correlated with fauna at sites for which absolute dates are available. They also estimated the antiquity of the specimens by molecular dating of DNA in a cellular organelle called the mitochondrion, because a higher percentage of the mitochondrial genome was covered by sequencing than was that of nuclear DNA (although similar date estimates were obtained using nuclear DNA from the two more recent specimens).
The mitogenome data revealed that the most recent of the three mammoth specimens, dubbed Chukochya, lived more than 680,000 years ago (for comparison, the iconic woolly mammoth, Mammuthus primigenius, first appeared in the fossil record around 700,000 years ago3). In a phylogenetic tree made using nuclear DNA, Chukochya fell outside a group comprising all woolly mammoths from the Late Pleistocene (129,000–12,000 years ago). This finding is consistent with the morphology of the Chukochya molar, which identified it as an early form of woolly mammoth.
The second-oldest mammoth, called Adycha, lived about 1.34 Myr ago during the Early Pleistocene (which spanned from about 2.58 million to 773,000 years ago). The authors found that Adycha belonged to a population ancestral to woolly mammoths, and which lived before Chukochya. There were substantial differences between the molar of Adycha and those of Chukochya and more-recent woolly mammoths, in terms of enamel thickness, number and density of enamel plates, and height of the crowns.
[...]
The most ancient mammoth was Krestovka, estimated by mitogenome dating to have lived about 1.65 Myr ago (although biostratigraphy suggested a slightly more recent date).
Elephant family tree
Source: Nature
A timeline of mammoth evolution. The ancestors of mammoths and Asian elephants (Elephas maximus) originated in Africa (indicated by yellow highlight). Both lineages migrated into Eurasia (red) after they had diverged (the date of this divergence is uncertain). Van der Valk et al.2 have sequenced DNA from three ancient mammoth specimens from Siberia, dubbed respectively Krestovka, Adycha and Chukochya. Genomic analyses found that Adycha and Chukochya were part of the lineage that gave rise to woolly mammoths. By contrast, Krestovka did not contribute to the woolly mammoth genome, but came from a lineage that diverged roughly two million years (Myr) ago, before migrating into North America (blue). Genomic analyses suggest that hybridization occurred between the Krestovka and woolly mammoth lineages, leading them each to contribute 50% to the ancestry of the North American Columbian mammoth (Mammuthus columbi). After woolly mammoths entered North America about 100,000 years ago, they interbred with the Columbian mammoths, replacing 12% of the Columbian mammoth genome. The timescale shown is based on genetic dating. (Figure adapted from Fig. 2c of ref. 2.)
This was not unexpected because to leave remains in Siberian permafrost, Adycha and Chukochya must already have had some adaptation to cold climates, however the Late Pleistocene woolly mammoths show evidence of further adaptation. For example, the gene TRPV3, involved in sensing low temperatures, shows more variation than in its Chukochya ancestor.
Krestovka, however, was show to not be directly ancestral to woolly mammoths, but ws the descendant of a line that diverged from the Adycha-Chukochya line some 2 million years ago and is probably ancestral to the North American Columbian mammoth (Mammuthus columbi), having migrated to North America some 1.5 million years ago via Beringia - a land bridge between Alaska and Siberia which existed when sea-levers were lower during glacial maxima.
And here it gets more complicated. The DNA of the Columbian mammoth is about half Krestovka and half woolly mammoth, indicating that when the Krestovka and Adycha-Chukochya mammoths got to North America, possibly in quite small numbers, they interbred an estimated 420,000 years ago.
In that case, the Columbia mammoth may be an example of a new species arising by hybridization.
I trust those Creationists who might be tempted to fall for the daft notion that the Theory of Evolution is about to be overthrown and replaced by the magical 'Intelligent [sic] Design' superstition, will have noticed how there was no sign of any such overthrow in this paper to which the TOE was the fundamental basis that made sense of the genetic evidence.
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