Tuesday, 11 October 2011

A Certain Sugar Molecule May Have Affected Human Evolution

Turkana Boy, a  nearly complete skeleton of the
now extinct Homo erectus, an early form of human which
was a dominant predator across the globe
Many different factors have affected human evolution. Climate change, interbreeding with Neanderthals and Denisovans have all made their mark upon our species. Now scientists from the University of California San Diego School of Medicine say that a certain kind of sugar molecule also changed the way we evolved. Sugar comes in many forms. Sucrose, more commonly known as table sugar, is used in sweet foods and glucose is the substance which is used by all lifeforms on Earth.

However there are less well known forms such as fructose, galactose or xylose. The particular sugar that the scientists studied was another bizarre example called sialic acid. These sugar molecules are found on the surfaces of cells where they aid chemical interactions with other cells. The acid comes in a series of different forms, each of which is used by different creatures. Millions of year ago, the various forms of ape shared a type of sialic acid known as N-glycolyneuraminic acid, better known as Neu5Gc.

Scientists believe that around 3 million years ago the malaria parasite caused a slight mutation in the genetic code of human ancestors which inhibited certain enzymes from creating the Neu5Gc molecule. As a result, humans produced another variant of sialic acid called Neu5Ac which is a primitive precursor to Neu5Gc. However this change only occured when humans became dominant predators.

'It's hard to be sure exactly what happened because evolution works on so many things simultaneously, but the change in sialic acid meant that early humans developed an immune response to Neu5Gc,' said Dr Pascal Gagneux, an evolutionary biologist and associate professor of cellular and molecular medicine at UC San Diego. It became viewed by their immune systems as foreign, something to be destroyed. At about the same time, they started eating red meat, a major source of Neu5Gc, which may have further stimulated the immune response.'

In mammals, pregnancy is a process which is so energy demanding, that it can be fatal for the mother. Therefore only the best suited sperm will fertilise the egg. Primarily it is a case of the first to the finish line. Sperm then pass through the uterus which is patrolled by vast numbers of antibodies, cells similar to the white blood cells which deal with foreign biological threats.

Anything foreign is targeted and destroyed which is why out of the millions of sperm which begin to make their way towards the egg, less than 20 will reach their goal and only one will fertilise the egg. As sperm are cells, they use the sialic acid coating to communicate with other cells. Some may have used the Neu5Gc acid from their diet of red meat, due to their new position as dominant predators. Therefore these Neu5Gc positive sperm would have been targeted and destroyed by the antibodies in the uterus.

We know that around 3 million years ago, one of the early humans, Homo habilis, began to hunt, scavenge and eat meat. Not only did this increase brain size and allow them to use tools, but this diet may have weeded out the Neu5Gc acid from their bodies thus reducing the chances of fertilisation by sperm carrying this particular molecule. By making research models using mice and ape sperm which carry different variants of sialic acid, scientists found that such a genetic change could have happened between the evolution of proto hominids and true members of the genus Homo.

Interestingly, they found that a 100% infertility rate was not necessary for the transition. Most importantly, a slight change could have affected the evolution of our immune system by changing antibody defence systems based upon a preference to different types of sialic acid.

This would be similar to updating the virus database for a firewall in a computer.'We suggest that the immune mechanism described here was involved in the origin of the genus Homo,' said study co-author Ajit Varki, MD, Director of the Center for Academic Research and Training in Anthropogeny at UC San Diego.