Evolution Made Easy

Evolution is the process of animals changing over time. They change simply in order to survive the changing conditions they live in. The way it works is through a process called Natural Selection. Brush up on this concept with some examples of natural selection.

Life's first recorded appearance on the planet was around 3.8 billion years ago. These were very simple bacteria. Since then more complex life forms have evolved, especially after the appearance of multicellular life at around a billion years ago. Thanks to the fossil record we have a lot of information about the development of life (how are fossils formed?).

Time

Time is broken up into periods based on big events. These could be big changes in the positions of the landmasses resulting from continental drift, but are most often associated with big changes to life on earth. For example, the extinction of the dinosaurs is used as a boundary between two labeled periods of time.

There are two ways of showing time passing from the past to the present. This can be vertical with the beginning at the bottom, or horizontal with time starting on the left and moving toward the present day on the right.

The lifespan of species is shown as lines, tree branches or bars. Related species are joined by these lines or branches.

It's a good idea to have the biology timeline at hand, as it's easy to get confused with the names of the time periods. For a long time I could not remember the difference between Oligocene and Eocene.

To get a good understanding of the development of reptiles it is also necessary to look at least briefly at the rise of the major animal groups through time.

The Evolution Of Fishes

Fish, including sharks, are the oldest group of vertebrate animals. They started off as very primitive creatures without jaws, and the descendents of some of these creatures still live today. Then they developed jaws, armor plating, lungs, fleshy limbs and a host of other features. Some of these were successful and some were not.

It was the fish with fleshy limbs, called lobe-finned fish, that gave rise to the amphibians and all later vertebrate groups. The most recent find is one that clearly links these fish with early amphibians. Tiktaalik posessed features that both define it as a fish and as an amphibian. It was also found in rocks that are older than those containing the oldest amphibians, but younger than those of the lobe-finned fish to which it is related.

The Evolution Of Amphibians

Early amphibians had the enormous advantage of being able to emerge from the water to move around and hunt, or perhaps they also used the land to escape from predators of their own. This caused a huge burst in the number amphibian species and they diversified to fill all available niches. Like the fish however, they were still bound to the water to breed.

The earliest land-going amphibians were still rather fish like in many regards. Two of the earliest known land vertebrates were Acanthostega and Ichthyostega. These creatures were present in the late Devonian period, approximately 365 million years ago. They represent the first known vertebrates that were truly built to cope with life at least partially on land.

There was a long period of overlap when both amphibians and early reptiles coexisted and competed. One giant amphibian from this time is Eryops, the largest amphibian of the time.

The Evolution Of Reptiles

Once the first reptiles freed themselves from the shackles of having to lay eggs in water they were free to cover and conquer the land. They exploded into an unbelievable array of forms and sizes and gave rise to all current reptiles, birds and mammals.

The Rise Of Mammals

With all the hype about dinosaurs, it's easy to overlook the fact that mammals evolved shortly after the initial radiation of reptiles. Their heyday was not to come until the dinosaurs became extinct and with them the majority of the reptile species on the planet.

The Rise Of Birds

Birds are the last group to emerge, having evolved from feathered dinosaurs about 135 million years ago. They enjoyed a brief stay in the "largest living land predator" slot during the Tertiary period.

Convergence Of Form

Throughout the fossilrecord and all around us today we see examples of how different groups have evolved to suit their environment. Along the way there are some startling similarities between unrelated animals that live in similar environments. This process is called convergent evolution. Also have a look at the crocodile fish for another fascinating example of this process.

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