Missing Link

Print More

(HOST) Sometimes science can seem to resemble one giant game of fill-in-the-blanks. And it’s a process that fascinates commentator Ruth Page.

(PAGE) Science has been finding “missing links” for years. For a time, folks skeptical about evolution said without THE missing link, ape-to-human, a creator was needed. Fossil finds in more recent years clearly show the gradual development of upright humans over more than a million years.

But when rock erosion formed land, how did sea plants and animals adapt? Well, blue-green algae can make food from sunpower, water, and carbon dioxide, they could have been the starters. But as soil formed, how could all gilled-and-finned animals, dependent on the ocean for life, ever adapt to land? That was a major missing link.

Fossils to the rescue again. Three hundred and seventy million years ago, there was a fish named Eusthenopteron, its fossil fins show primitive forerunners of a foot with eight toes. A million years later came Ichthyostega, which still had gills and fins, but the fins covered toes and leglike bones strong enough to support the upper half of their bodies. Compared to an ordinary fish, that’s progress.

Later discovered in Arctic Canada was an animal called Tiktaalik, now proposed as THE missing link between sea and land animals. It is a fish-tetrapod, or fish with legs and feet in its fins. Its forerunner was Acanthostega, a gilled fossil fish from Greenland, which showed primitive “legs” and “feet” in its fins. Scientists speculate that such creatures could navigate swampy waters, which were their habitat. The appendages would let them adapt to muddy shorelines by pulling or pushing themselves along. Perhaps they could even push across river beds. They lived around ancient coasts that straddled the equator, lands that later moved north and became North America and Europe.

Tetrapods all have necks, unlike fish. And Tiktaalik was a fish with a neck. It could raise its head out of the water and breathe air when it ventured onto coasts. The strong fin-feet were able to support the front half of its body. Scientists point out that animals like us, that have legs, feet and necks, originally evolved in water (and thank goodness, ’cause that’s all the planet offered several billion years ago).

In the ancient land climate, deciduous trees evolved and bacteria that feasted on their dropped leaves and other remains used up most of the oxygen in local water. Thus fish that ventured onto the shores and could breathe air gained an advantage over their water-bound cousins, by having two sources of food.

Over time, these air-breathers diversified, evolved further, and the vast variety of early land-bound creatures came to be.

It seems that that link, sea-bound animals to land animals, is no longer missing.

Ruth Page has been following environmental issues for twenty years. She is a long time Vermont resident and currently lives in Shelburne.

Comments are closed.