How Science Works #2…

The theory of evolution attempts to explain the many connections between all living things on Earth. The fossil record, evidence from genes, geographical distribution and laboratory experiment all point towards living things sharing a common ancestor from about 3.5 billion years ago. Simple, self-replicating molecules may have become more complex bacteria-like organisms from which all subsequent life has evolved. The exact mechanism by which simple organic molecules became fully functioning organisms (abiogenesis – life from non-life) is not understood, but progress is being made slowly unravelling the mystery. Once life started, natural selection could act upon populations to produce the diversity of life we see today. To find out more read this post on evolution from December 2009. There is also a nice little video on the origins of life.

In my previous How Science Works post I said that a good theory should have predictive power. Evolutionary theory has just this ability as demonstrated by Neil Shubin and his team’s discovery of Tiktaalik in 2004. Tiktaalik is a fish that lived about 375 million years ago in the late Devonian period. It is what is known as a transitional form because it shares characteristics of both fish and early amphibians. It lived in warm, shallow water, with low oxygen levels, and had developed leg-like front fins which helped it to move around near the water’s edge. Like a mudskipper, it may have been able to make brief forays onto land, although it still relied upon gills for most of its gaseous exchange. It was starting to show some of the bone structures now found in land animals; wrist bones, an enlarged ribcage, primitive lungs, a flexible neck. At the same time it still had scales, fins and gills like a modern fish. It was a great find and is a fascinating creature.

What I like about the discovery is how it was achieved. Shubin and his team reasoned that, were evolutionary theory true, there must have existed at some point a creature that was transitional between fish and amphibians. They predicted what conditions it would need to live in, and when it must have existed, based upon the existing fossils of fish and amphibians.

400 million years ago the land masses of Earth looked very different. They existed as a supercontinent known as Pangaea. To find the Tiktaalik remains, Shubin had to predict where on Pangaea the correct conditions would exist for a fish-to-amphibian transitional form to evolve. He then had to calculate where those rocks, potentially holding the fossils he was looking for, would be now after 375 million years of tectonic movement. His team settled on Ellesmere Island, between Canada and Greenland, with the Arctic Ocean to its west and the Baffin Sea to its east. They sent an expedition to dig into layers of rocks of the right age and did not initially find what they were looking for. They mounted a second expedition the following year and uncovered the fossilised skeleton of the animal subsequently named Tiktaalik roseae.

Questions…

1. What is meant by a fair experiment?
2. What are the five different kingdoms of organisms?
3. For one of the five kingdoms, describe the characteristics of the group.
4. How long ago, roughly, did the first reproducing organism appear on Earth?
5. How is abiogenesis different from evolution?
6. What would be a good definition of evolution as it appears in the Theory of Evolution?