Human family tree
February 12, 2009 - 200th anniversary of the birth of British naturalist Charles Darwin, who formulated the theory of evolution as a result of observations made on a five-year surveying expedition on board the ship HMS Beagle.
When Charles Darwin’s “On the Origin of Species” appeared in 1859 he knew nothing of genes and DNA, it would take nearly a century more before James Watson and Francis Crick deciphered the structure of the molecule that contains the blueprint for building an organism.
Now, however, Darwin’s theory of adaptation by natural selection -- the process by which organisms change to better fit their environment -- is shaping genetic research today.
For years researchers have puzzled over whether adaptation continues to play a major role in human evolution or whether most changes are due to neutral, random selection of genes and traits.
A study published last year by Henry C. Harpending of the University of Utah and John Hawks of the University of Wisconsin–Madison found that over the past 10,000 years humans have evolved as much as 100 times faster than at any other time since the split with our earliest human ancestor some 7.2 million years ago.
Another study by Pardis C. Sabeti of Harvard University and her colleagues looked for signs of natural selection across the human genome.
“We are currently finding the genetic variations that have arisen through evolution to enhance survival and reproductive success,” Sabeti said. “These include variations that protect from infectious disease, act against a changing environment and sun exposure, or help in metabolism with changing diets.”
As recently as 6,000-12,000 years ago, the evolution of a gene for skin colour resulted in the lighter skin shades in populations at northern latitudes. Lighter skin enabled humans to capture more sunlight to make Vitamin D.
Other examples include resistance to Lassa fever virus and partial resistance to other diseases, such as malaria, among some African populations.
Sabeti’s research is giving some insight into how certain traits develop over time and in certain regions. For instance, dairy herding arose in the past 10,000 years -- a period corresponding to just 400 or so human generations -- over which time lactose tolerance developed rapidly in some humans.
“All mammals drink milk from their mothers when young,” Sabeti explained, “but a lifelong tolerance for milk wasn’t historically normal”.
In 2002, a mutation was traced in the DNA that controls production in the intestines of the enzyme lactase. This adaptation enabled Europeans to rely on cows and milk products over time, said Sabeti.
More recently, different mutations affecting the same gene were found to predominate in East African and Saudi Arabian populations who traditionally herd milk-producing goats and camels, illustrating how culture can reinforce the forces of evolution.
“Common genetic variations that have arisen relatively recently are clear signs of the work of natural selection,” says Sabeti. “But we are only at the beginning of this long and exciting process.”