|The History of Life on Earth|
Organisms move onto land
During the period 500 – 450 million years ago, the rise of the fish occured. This was the first evidence of the first vertebrates. These fish are called ostracoderms and were slow, bottom-dwelling animals that were covered from head to tail with heavy armour of thick bony plates and scales. Ostracoderms had no jaws and had poorly formed fins thus were slow and covered in heavy armour to protect themselves from other predators.
Ostracoderms are important organisms in the history of life on earth because they were the first organisms to have a backbone - they were vertebrate. Another innovation of ostracoderms was the use of gills not for feeding, but exclusively for respiration. In all previous life before them, gills were used for both respiration and feeding. Also, every other animal today can be traced back to have significant link with these fishes. Most scientists believe that the ostracoderms gave rise to jawed fish with backbones, and then they in turn gave rise to amphibians. Amphibians are vertebrates that have legs and live both on land and in water.
Evidence of land dwelling organisms at that time was the discovery of wax-coated algae that was found on land dating back to about 430 million years ago. To support this, millipedes were found dating back to about 420 million years ago. Millipedes evolved to be the first land animals. They belong to a group of Myriapoda. Myriapoda has four classes, which are centipedes, millipedes, pauropods, and symphylans. They all have a similar body plan consisting of a head followed by an elongate trunk with many legs. All Myriapods are relatively rare in the fossil record, a result of a light and thin cuticle shell, and their existence in non-marine environments where fossilization is less likely. The few older fossils, however, are testament to the ancient appearance of these Arthropods.
The oldest fossil that has been found is a millipede of species Pneumodesmus Newmani which is a terrestrial oxygen-breathing organism from the mid Silurian dating to about 420 million years ago. It exhibits cuticular openings that prove that it lived in an oxygen atmosphere. These openings would have been spiracles or atmospheric oxygen intake organs suited for the environment. The one-centimetre millipede was prised out of a siltstone bed dating about 420 million years ago.
At 380 million years ago, vertebrates start to move onto the land. These vertebrates were mostly amphibians. It was a transition from fins to feet that took place in the water. This is one of the most amazing things in the history of life on earth because of the evolution of fish into walking land animals. The very first few amphibians seem to have developed legs and feet to scud around on the bottom in the water, as some modern fish do, though don’t walk on land. This aquatic-feet stage meant the fins didn't have to change very quickly, the weight-bearing limb musculature didn't have to be very well developed, and the axial musculature didn't have to change at all. Eventually, of course, the amphibians did move onto the land.