A number of genetically inherited disorders such as psoriasis, sickle cell anemia and Chrohn’s disease have been a part of humanity for millions of years. In a new study, researchers are examining the idea that these disorders may have some benefit, either now or in the past, that has caused them to remain part of our genetic makeup.

People have a tendency to think of evolutionary traits in absolute terms. A specific trait is considered good or bad, without regard to context and without the idea that in some cases there could be a trade off.

Scientists now believe that some genetic conditions such as psoriasis, a chronic skin condition, have been with use since a time before Neanderthals and Denisovans and probably arose in a common ancestor shared by both of those groups and Homo sapiens.

“Our research shows that some genetic features associated with psoriasis, Crohn’s disease and other aspects of human health are ancient,” said Omer Gokcumen, PhD, in a statement.

Gokcumen is a University at Buffalo assistant professor of biological sciences and one of the authors of a new study published in Molecular Biology and Evolution.

According to the researchers, some of our ancestors had deletions, a telltale feature of the disorder, while others did not. Not only did they share the genetic predisposition but shared it in roughly the same proportions that we see today.

One possible explanation for this is that, while it may not be immediately obvious, there may be some genetic advantage to conditions such as Chron’s and psoriasis.

Gokumen points to sickle cell anemia as a prime example of a trade off in genetics. As a disorder it causes red blood cells to take on a crescent-like shape which leads to anemia. At the same time that curved shape helps to prevent malaria by keeping the parasites out of cells. Because of the benefit, the gene would be retained and passed on in areas where malaria is prevalent.

The researchers suggest that a similar trade off may be at work with Crohn’s disease.

“Crohn’s disease and psoriasis are damaging, but our findings suggest that there may be something else — some unknown factor now or in the past — that counteracts the danger when you carry genetic features that may increase susceptibility for these conditions. Both diseases are autoimmune disorders, and one can imagine that in a pathogen-rich environment, a highly active immune system may actually be a good thing even if it increases the chances of an auto-immune response,” said Gokcumen.

Yen-Lung Lin, a PhD candidate in UB’s Department of Biological Sciences and lead author of the study says that “opposing evolutionary pressures” may be “under appreciated”.

“We’re thinking forces that maintain variation might be more relevant to human health and biology than previously believed,” says Lin.

The researchers compared modern human genomes to those of other related species including chimpanzees, Neanderthals and Denisovans. They found chunks of DNA in chimpanzees that were erased through the process of evolution. These segments, called deletions, are present in some modern human genomes but not in others.

The team found that important deletions that vary among humans likely dated back a million years or more, originating in a common ancestor of Neanderthals, Denisovans and Homo sapiens. These deletions included psoriasis and Crohn’s disease patients. They also found deletions which are linked to individual responses to a number of drugs including human growth hormone.