Birds to Reptiles

Because reptiles appear in the fossil record before birds, we can guess that the common ancestor of birds and reptiles was an ancient reptile, and would have looked like one. We now know that this common ancestor was a dinosaur. Its overall appearance would give few clues that it was indeed a “missing link”—that one lineage of descendants would later give rise to all modern birds, and the other to more dinosaurs. Truly birdlike traits, such as wings and a large breastbone for anchoring ...

 Missing Links Make Defining Species Possible

As we trace the ancestry of modern Homo sapiens backwards, there must come a time when the difference from living people is sufficiently great to deserve a different specific name, say Homo ergaster. Yet, every step of the way, individuals were presumably sufficiently similar to their parents and their children to be placed in the same species. Now we go back further, tracing the ancestry of Homo ergaster, and there must come a time when we reach individuals who are sufficiently different fro...