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mammoth skull

Glen M. MacDonald

Some 45,000 years ago, the mighty woolly mammoths thundered across Beringia, a region that included parts of present-day Siberia, Alaska, and the Yukon, as well as the land that bridged what is now the Bering Strait. On the fertile steppes they grazed on nutritious grasses and willows. Yet by 4,000 years ago they had vanished, the cause of their demise a contentious topic of debate.

To examine the factors that may have been fatal for mammoths, geographer and paleoecologist Glen M. MacDonald of the University of California, Los Angeles, and colleagues gathered unprecedented amounts of radiocarbon and mitochondrial-DNA data on mammoth fossils; evidence of ancient humans and of prehistoric temperatures; and fossilized remains of primeval peatlands and forests. Overlaying that data on a series of maps of Beringia, the team visualized how mammoth and human populations grew, shrank, and relocated in relation to changing environmental conditions.

Beginning with the last ice age, 25,000 years ago, harsh conditions reduced the sizes of mammoth populations in the north, possibly claiming one genetic line, but led to a growth of mammoth populations to the south, in Siberia’s interior. By 15,000 years ago, northern mammoths made a small comeback on more hospitable grasslands. However, southern Siberian populations increasingly encountered marshy peat bogs, thick conifer forests, and toxic birch woodlands, environments presumed hostile to mammoths. That trend continued for mammoths across Beringia before and after a 1,300-year cool period known as the Younger Dryas, nearly 13,000 years ago. Quickly rising temperatures after the Younger Dryas enabled peatlands, birch, and conifers to dominate the landscape. Facing troublesome terrain and scarce nutrients, mammoths also suffered greater hunting pressures as human populations grew and people crossed the land bridge to North America. Between 10,000 and 4,000 years ago, mammoths were dying out, the last ones finally succumbing on remote Arctic islands.

The study concludes that multiple, intertwining stressors led to the behemoths’ extinction: a warming climate, habitat changes, and human pressure—familiar threats for many species today. (Nature Communications)