Wednesday, 27 October 2010

Atavisms: A blast from the past

The evidence for evolution comes in a variety of forms, many of which can only be understood with a good grasp of  modern concepts in  biology, genetics, ecology, molecular biology, developmental biology or geology. However, there are examples which can be seen with the naked eye and can be understood by anyone. For me, this is one of the best examples around...

In the natural world, traits which have been lost through evolution can reappear naturally. These are known as atavisms, and they are one of the most visually compelling evidences for evolution.

Here are a few examples:

 Human Tails
Most primates have long tails which can be used for balance or even serve as a fifth limb, which suits their arboreal lifestyle. However, all apes have lost their long tails through evolution, yet we still have the genes for a tail- they just aren't expressed anymore. This is because other genes override the expression of a tail during development. However, every so often a mutation may occur which permits the development of a tail.
Over the years, there been over 100 reports of human tail atavisms. These are not just stumps or extra bits of tissue; instead they have vertebrae, and some people can even move them!

Hind limbs in dolphins and whales
Dolphins and whales are both mammals. Since mammals evolved on land, the ancestors of dolphins and whales must have also lived on the land. During their transition to a marine lifestyle, they have lost their hair and their hind legs and develop a more streamlined shape. Occasionally these features reappear in the wild. In October 2006, this four finned dolphin was donated to the Taiji Whaling Museum after being captured off the west coast of Japan by fishermen.

Horses with extra toes
 Horses are actually standing on their hooves with the same bones as a human would if they were standing on tip toes. This is because although they evolved from a multi-toed creature,  these extra toes have shrank in size over millions of years. Now all that remains of them are a few vestigial bones. There are a  few recorded examples of extra toes in horses, such as "Clique, the horse with six feet", which was donated to Yale after it's death in 1891. Complete ulnas and fibulas have also been found in Welsh and Shetland ponies as well as miniature horses.

Chickens with teeth
This atavism has been induced by scientists by using viruses that mimic a known mutation which has the potential to cause chicken embryos to grow teeth. Birds evolved from dinosaurs, and we know that dinosaurs had teeth. In fact, even early birds such as Archeopteryx had teeth. Scientists can induce  this atavism because the chickens still retain the ability to grow teeth, but it is overridden by a complex set of developmental pathways. Unfortunately, the pathways are so complex that tinkering with them can often be lethal for the embryo. This tells us that (a) changes in natural populations are far more subtle than those in the lab, and (b) we are only skimming the surface of a science that has great deal of practical potential.

1 comment: