to identify natural selection as the driving force behind evolution, or what Darwin termed “descent with modification.”

When Darwin published On the Origin of Species in 1859, paleontology was still a rudimentary science. Sedimentary rocks from many time periods were unknown or had been inadequately studied. Darwin spent almost 20 years gathering evidence that supported his idea before making it public, but he also carefully considered evidential problems for his view, such as the inadequacy of the fossil record and the rarity of intermediate forms between some major groups of organisms at that time.

A near complete skeleton of a transitional bird-like fossil that was discovered in China and reported in 2006.

In the century and a half since then, paleontologists have found many intermediate forms that were not known in Darwin’s time. In a variety of locations, sedimentary rocks that are between 540 million and 635 million years old contain traces of soft-bodied multi-cellular organisms, and fossilized tracks in earlier sediments hint at the existence of wormlike creatures as long ago as 1 billion years. Some of these organisms are likely to be the intermediate forms between the single-celled organisms that were Earth’s sole inhabitants for the first 2 or more billion years of life’s history and the hard-bodied organisms that appear in abundance in the fossil record beginning about 540 million years ago. Similarly, many of the organisms that appeared during this period were transitional forms between earlier soft-bodied organisms and major evolutionary lineages such as the fishes, arthropods, and mollusks that have survived to the present day.

As described at the beginning of this document, Tiktaalik is a notable transitional form between fish and the early tetrapods that lived on land. Fossils from about 330 million years ago document the evolution of large amphibians from the early tetrapods. Well-preserved skeletons from rocks that are 230 million years old show dinosaurs evolving from a lineage of reptiles. A long-standing example of a transitional form is Archaeopteryx, a 155-million-year-old fossil that has the skeleton of a small dinosaur but also feathers and wings. More birdlike fossils from China that are about 110 million years old have smaller tails and clawed appendages. In the more recent fossil record, the evolutionary paths of many modern organisms, such as whales, elephants, armadillos, horses, and humans, have been uncovered.

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