What Caused Early Humans to Rapidly Evolve One Million Years Ago?

There are certain segments of DNA that have remained relatively unchanged in the tens of millions of years since mammals split from their reptilian forebears— tried and true segments of code that remained unchanged throughout the course of mammalian evolution. However, roughly one million years ago a sudden change in these normally well-preserved sequences occurred in the ancestors of modern humans that set us on the road to becoming a species markedly distinct from the rest of the animal kingdom—even from our closest genetic cousins, the chimpanzees.

First discovered in 2006, these segments of genetic code, called human accelerated regions (HARs), are shared across numerous species of mammal, and have remained relatively unchanged for millions of years. However, about one million years ago 49 of these genes inexplicably underwent a rapid series of changes, bringing about a remarkable evolution in our ancestors, such as an increase in brain size and our ability to walk upright.

Recently, a major clue behind the mechanism behind this change was discovered: the way that these early humans’ DNA was folded saw an abrupt change, allowing “hijacked” enhancers, molecules that increase the ability for the genetic code to be copied for use in the organism, to access otherwise unrelated sections of code.

The information that a given strand of DNA can impart depends not only on the information encoded within the strand, but also how that information is presented through the way the strand is stored within the cell: since a typical strand of human DNA is a complex molecule roughly 2 meters (6 feet) long, it has to be tightly folded to fit in the tiny nucleus of a cell. This folding means that certain portions of the code are accessible on the outside of the folded structure, ready for use by the cell’s transcription mechanism, while the code folded away deeper within the structure remains unreadable.

These suddenly-accessible portions of the code are associated primarily with human embryo development, particularly with neurological skills—mental focus, intelligence, memory, social skills—traits that are far more developed in humans than they are in other mammalian species. Other changes are associated with the evolution of the opposable thumb, while others modify the ankle and foot, enabling our status as a bipedal species.

One peculiar aspect of this sudden change in gene expression is that the rest of the DNA itself would have to have adapted just as quickly to what is expected of the organism: in one evolutionary moment the DNA is saying one thing; in the next, the individual cells are hearing an entirely different tune.

“Imagine you’re an enhancer controlling blood hormone levels, and then the DNA folds in a new way and suddenly, you’re sitting next to a neurotransmitter gene and need to regulate chemical levels in the brain instead of in the blood,” explains study lead Katie Pollard, a computational biologist and director of the Gladstone Institute of Data Science and Biotechnology.

“Something big happens like this massive change in genome folding, and our cells have to quickly fix it to avoid an evolutionary disadvantage.”

What prompted this sudden change in the folding of our DNA—along with how our ancestors’ cellular biologies coped with the new genetic instructions—remains a mystery, although Pollard and her team hope to shed some light on the enigma of human evolution as their research progresses.