paleoanthropology, evolution and genetics

Photo Credit: Hillside above the Rising Star cave system, South Africa. John Hawks CC-BY-NC-ND

Notable paper: Bos, K. I., Harkins, K. M., Herbig, A., Coscolla, M., Weber, N., Comas, I., ... & Krause, J. (2014). Pre-Columbian mycobacterial genomes reveal seals as a source of New World human tuberculosis. Nature, 514(7523), 494-497. doi:10.1038/nature13591

Synopsis: The authors sequenced bacterial genomes from three 1000-year-old Peruvian skeletons to determine the source of their tuberculosis bacilli. These genomes were most similar to those from seals and sea lions, even though the modern New World and Old World tuberculosis strains are clustered together with each other (and with the chimpanzee bacillus strain). The implication is that ancient Americans came across the Bering land bridge without tuberculosis, and acquired it as a zoonosis from pinnipeds in South America.

Important because: The close relationships of present strains throughout the New World with European strains challenged the interpretation of ancient skeletal remains with tuberculosis. Bos and colleagues show that these modern strains probably replaced earlier strains that had existed in the Pre-Columbian New World.

Problematic because: The paper suggests the common ancestor of all these bacilli -- including the non-human strains -- lived around 5000 years ago. Transoceanic tuberculosis transfer by seal does make it possible to get the disease to the Americas in that time frame. But this estimate for the age of the common ancestor is an order of magnitude more recent than previous estimates for just the human strains. It conflicts with skeletal evidence for tuberculosis in even earlier specimens. The paper does defend its high mutation rate estimate in several ways, but I found it puzzling that the authors did not do more to explain why their estimates conflict with so much previously-published evidence.