August 23, 2012

Arms, Legs And Other Phenomena

The other day, as I was struggling home from the supermarket with my shopping cart in the rain, I thought to myself, “If there were such a thing as Intelligent Design, I would have been born with wheels, eight arms and an umbrella growing out of my head.”

The reason that I have two arms and two legs is that I have descended from fish with two sets of paired fins – pectoral fins and pelvic fins.

All terrestrial vertebrates (amphibians, reptiles, birds and mammals) have this same four-limbed structure because they shared the same evolutionary path.

In humans, pectoral fins have become arms and pelvic fins have become legs.  Paired fins have becomes two pairs of legs in frogs and horses. Birds and bats both have wings, although bird wings and bat wings are very different, structurally.

The front limbs of dolphins and whales have become flippers. Although dolphins and whales don’t seem to have hind limbs, when they are in the womb they have hind limb buds. Sometimes, a whale or a dolphin will be born with remnants of hind limbs.

Most snakes have lost all of their limbs.  Pythons and boa constrictors, however, have vestigial hind legs, which are known as “pelvic spurs”.  There are legless lizards; these still have parts of the collar, shoulder bones and pelvic girdles to which their ancestors’ limbs would have been attached.

The limbs of tetrapods (four-limbed land vertebrates) are adaptations of the limbs of lobe-finned fish. These are fish that have fleshy lobes – bony, muscular stalks – at the bottom of their fins.   Lobe-finned fish can swivel these lobes about in sockets the same way we can swivel our arms and legs.

Having lobed fins created an evolutionary advantage for lobed-finned fish. If a pond or stream dried up, the fish could walk across land to another pond or stream.

What ever happened to lobe-finned fish? Most species have become extinct since they came into existence about 400 million years ago. However, two kinds of lobe-finned fish still exist today – coelacanths and lungfish.

Most fish today do not have these bony lobes.  These fish are known as ray-finned fish.

Scientists have identified genes that have been associated with the development of fins in fish and limbs in land animals.

In 2010, researchers discovered that they could get embryos of ray-finned fish to develop lobed fins by removing some of their genes. This indicates that the development of lobed fins – and later, limbs – came about as part of the accidental loss of some genes. Since the ability to walk on land enables vertebrates to exploit new environments and so enhances their ability to survive, the mutation persisted.

The paddlefish has often been used to study the genetic aspects of vertebrate fin and limb evolution. The paddlefish, which is related to the sturgeon, is part of an ancient group of fish known as Chondrostei, which could be found on Earth during the Triassic period, more than 200 million years ago. Because of its ancient lineage, it has been used as a substitute for the ancestor of lobe-finned fish in genetic studies.

Recently, researchers discovered that  paddlefish genes were duplicated about 42 million years ago.

Whole gene duplication is an important evolutionary event, because it allows changes to take place without the loss of essential functions. Gene duplication is associated with the evolution of new species. It is associated with the evolution of jawed vertebrates – all of the vertebrates that are alive today, except for lampreys.

Since lobe-finned fish were on Earth more than 400 million years ago, according to fossil evidence, this suggests that fin and limb evolution might have taken somewhat different paths than was previously thought.

Image Credit: Vasilchenko Nikita / Shutterstock

Facebook Twitter Pinterest Plusone Digg Reddit Stumbleupon Email