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WORLD OF REPRODUCTIVE BIOLOGY

Two new studies tracing the origins of the Y chromosome challenge a long-standing notion in the field of human evolutionary genetics that the most recent common male ancestor of Homo sapiens is much younger than his female counterpart. Large regions of the Y chromosome do not recombine with the X chromosome and so can be used to generate phylogenetic trees based on the timing of appearance of single-nucleotide polymorphisms (SNPs). Researchers can calibrate the SNP ‘‘clock’’ by comparing the genetic findings with archeological records of key human migration events that established new Y-chromosome lineages. The result is a rough estimate of when various Y-chromosome types emerged. Previous analyses of the Y chromosome have been limited to examining a relatively small number of individuals from a few populations, but advances in sequencing technology are now allowing larger-scale analyses. G. David Poznik et al. [1] sequenced the complete Y chromosomes of 69 males from a globally diverse population and calibrated their clock using major migration events into the Americas. They then applied the same analytical techniques to previously sequenced mitochondrial DNA to estimate when mitochondrial ‘‘Eve’’ was on the planet. Their analysis suggests that the most recent common male ancestor emerged between 120,000 to 156,000 years ago, and the female ancestor between 99,000 and 148,000 years ago—approximately the same time frame. In a separate study, Paolo Francalacci et al. [2] sequenced a large non-recombining region of the Y chromosome (about 9 megabases) in a diverse population of 1,204 males from Sardinia, calibrating their clock using known migration events to that island. They concluded that our most recent common male ancestor was alive between 180,000 and 200,000 years ago. The findings suggest that our common male and female ancestor both lived in roughly the same time period, which is in sync with other emerging studies that overturn the notion of a very large age gap. The new results will aid in analyzing the timing of human migration events, such as the emergence out of Africa, which Francalacci et al. place at 110,000 years ago. That date is in accordance with the analysis of fossil remains, but is at odds with mitochondrial DNA studies that place this event closer to the present.