Katherine Pollard

1972 - present

"...you do not need to change very much of the genome to make a new species.”

Katherine Pollard heads the Pollard Group at the Gladstone Institutes at the University of California, San Francisco. She is Professor of Biostatistics and applies her skills to statistical analyses of the genomes of humans and other species.

One of the seminal questions I pursued in my research is to find out exactly what makes Homo sapiens different from our predecessor species. That might point the way to our future evolution into a successor species. Since all of our direct predecessor species are extinct, we have only fragmentary fossil comparisons. We're not even sure exactly which species was our direct predecessor. It was probably Homo heidelbergensis or Homo erectus or some close relative of one of those.

The best way to answer this question would have been to compare the genomes of Homo sapiens to the predecessor. Unfortunately, we have not yet been able to recover any DNA from the possible predecessor fossils. Svante Pääbo and his team at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany has been able to recover and analyze the genomes of two other extinct human species: Neanderthals and Denisovans. Although their comparison to Homo sapiens is informative, since neither of these species is in our direct lineage, it does not fully answer the question.

Katherine Pollard and her team have taken another genome related approach to answering this question. We can do genome analyses on living species that share common ancestry with Homo sapiens and compare them to our genome. Using massive computer power and statistical analyses we can infer a great deal from such comparisons. Genetic mutations are random so there is a baseline number of expected changes that should occur throughout the genome of all species. However, since natural selection is operating on those changes, some parts of the genome will show more changes than others because those changes conferring a beneficial effect will be selected to be passed on to future generations. These so-called “accelerated regions” tell us a lot about evolution and speciation.

Katherine Pollard and her team did such a genome comparison. She found that there is a very small part of the human genome that has undergone the most rapid changes since we split from the chimpanzees. That region is called HAR1 (human accelerated region 1). HAR1 consists of just 118 nucleotides (out of three billion in our genome). By looking at these same nucleotides in the genomes of chickens, chimps, and other species, she could determine that those same nucleotides were stable in the previous 300 million years in those other species. Something happened in the 5.4 million years since we split from the chimps to cause the changes in these particular nucleotides to be selected. Those changes must be related to the emergence of Homo sapiens. It turns out that HAR1 is important in regulating the genes that cause the human brain to develop the convolutions in our neocortex. It is these convolutions which provide the enormous increase in surface area of our brain enabling much more complex behavior. They are epigenomic changes.

There were some other HARs that Dr. Pollard examined as well. These are related to our linguistic ability and our increased manual dexterity—two other key differentiators of our species. Thus, it appears that Homo sapiens emerged because of a relatively small number of genetic mutations in a relatively short period of time. As Dr. Pollard states, “In other words, you do not need to change very much of the genome to make a new species.” This was a key clue to looking for the answers

Click on links to other players in my journey below.