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The first known birds. At some point in the evolution of birds from reptiles, there must have been various kinds of birdlike reptiles. Such creatures would have structures that are like between reptile scales and bird feathers. However, no fossil of any such birdlike reptile has yet been found.
The earliest bird fossils belong to a group called Archaeopteryx. Archaeopteryx lived about 140 million years ago. It looked like a reptile in many ways. However, it had feathers like those of modern birds. The first Archaeopteryx fossils were found during the mid-1800's. Altogether, seven fossils--six partial skeletons and a single feather--have been found. Six of the fossils were found in limestone beds in southern Germany. The best-preserved of these specimens shows the imprint of almost all the skeleton plus the flight feathers.
Unlike modern birds, Archaeopteryx had teeth, a reptile like tail, and three claws on each wing. It may have used these claws to climb trees. Scientists believe Archaeopteryx could fly, but probably only weakly.
The next well-preserved bird fossils date from about 90 million years ago. The best-known birds of that time are Hesperornis and Ichthyornis. Both were water birds that lived in what is now the Midwestern United States. At that time, a large Inland Sea covered most of the region. Hesperornis and Ichthyornis almost certainly ate fish. Like Archaeopteryx, both had teeth. But in other ways, they looked
more like modern birds than the Archaeopteryx. Hesperornis somewhat looked like a modern grebe or loon. It could not fly but was a strong underwater swimmer. Ichthyornis could fly and looked somewhat like a small gull.
When animals appeared on the earth. Most scientists believe that the earth formed as a planet at least 41/2 billion years ago. The first life forms were simple, single-celled organisms that appeared about 1 billion years later. More complex animals and plants gradually evolved from these simple organisms. Many groups of invertebrates arose about 650 million years ago. The first vertebrates--fish--developed about 500 million years ago, and the first mammals appeared more than 200 million years ago.
Another way of looking at these times is to imagine the history of life on the earth in terms of a single year. Start with the formation of the earth on New Year's Day, January 1. Bacteria, the first types of living things, would not appear until March 22. Many invertebrates would not show up until November 9. Fish would evolve from their invertebrate ancestors about November 20. Mammals would appear on December 16. Monkeys and apes would not be found until December 28. Human beings would appear only a few minutes before the end of the year, on December 31.
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