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.

Can we bring the Dinosaurs back?

This weekend I spotted some Dinosaurs by the Blackpool lights. Unfortunately they're not real since the only living descendants of Dinosaurs today are Birds. But the worlds most famous Dino-hunter, Jack Horner, is helping the man who he thinks has the potential to change all this. His name is Hans Larsson. His work involves merging paleontology and molecular biology in order to understand major evolutionary changes. One of his goals it to 'switch on' dormant genes or change the regulation of currently active genes so that ancient traits, lost through evolution, will be expressed. Hans Larsson believes that although birds have lost many dinosaurian traits, it may be possible have them expressed once again.

So far, Larsson and his postdoctoral assistant have began working on chicken embryos. Traits such as a long tail have been lost in birds, and we can see the gradual reduction in tail length in the fossil record. Larsson found that although the tail grows very well in early development, "at a particular stage in development everything comes crashing to a halt". They found that retinoic acid, which stimulates the release of an protein called sonic hedgehog, “pushed tail growth to the upper range of normal development. It had some effect, but it didn’t break it out of the cycle.” This gave an Larsson an idea of how complex the growth of the tail is. Unfortunately there isn't much research on how to sustain growth in a tail, so Larsson and his assistant have had to do everything from scratch.

“The experiment I’m envisioning is that you have a single embryo developing in the egg with multiple injection sites and multiple kinds of molecules to be really fine-tuning the regulation of genes,” says Larsson. “We’ll be able to inject different parts of the embryo at different times of development with different things. If we do that, if the timing and position are correct, we should be able to manipulate lots of different kinds of morphologies—feathers, wings, teeth, tails.

“It would take just a little bit of time to work out each one of those systems in very great detail, which we’re now doing for the tail. Other people are doing it for the limbs for clinical work. And teeth are being worked out by other people for mammals and such, and then we can just sit down and play with all these in concert, which has never been done before.”

This'Chickenosaur' may resemble something from the coelurosaurs, a clade of theropod dinosaurs which resemble birds more than carnosaurs.“Birds are dinosaurs, so technically we're making a dinosaur out of a dinosaur. The only reason we're using chickens, instead of some other bird, is that the chicken genome has been mapped, and chickens have already been exhaustively studied. A number of people in a number of different places are moving forward with the project slowly and carefully,” says Horner.

Below is a video by Hans Larsson about the work he does.