Here's a little essay I wrote a while back:
Stephen Jay Gould is famous for arguing that if we "replayed the tape of life" nothing even remotely similar to Homo Sapiens would emerge. He draws this conclusion from the infamous Cambrian explosion, in which many novel body plans were evolving. Some of the Cambrian fossils have an eerie other worldly look, and it is clear that if they had survived rather than Pikaia, the ancestor of all vertebrates, life on earth would be unimaginably different. Evolution is always contingent upon the step which precedes it, bulding upon and expanding what is already there. If this crucial and basic step towards vertebrate life were removed, it would follow that all the steps built upon it would cease to exist.
Critics have pointed out cases of convergent evolution and argued that if we did replay the tape of life a second time, much, though not all, would remain the same. This is wrong. Eyes, Wings, and Fins might still emerge, since they have emerged in all manner of lineages, even nonvertebrates. Yet take a look at the most famous example touted by the likes of Kenneth Miller and Daniel Dennett: The Marsupials. While it is true that striking examples of convergence exist between placental mammals and marsupials, the marsupial "mole", for instance, Stephen Jay Gould's main point remains unrebutted. The earliest mammals date to about 220 million years old, while Australia separated from Gondwana about 100 million years ago. Thus, mammals were divided geographically after 100 million years of evolution from their most primitive forms. The earliest steps in the history of placental and marsupial mammals were the same, and after they divided their evolution was (very roughly) similar. Contrast this to Stephen Jay Gould's argument that if the first step in the history of life was radically different, the steps following that would be even moreso.
Another objection to Gould's argument is that he assumes the success of the Cambrian groups as random. How do we know that there was a giant lottery going on? Perhaps it was not the luck of the draw, and there is a perfectly good reason Pikaia lived. This may be so, but consider this: If conditions were good enough for Pikaia's competitors to evolve and flourish for a time, need they be radically different for those competitors to have won the grand lottery? Secondly, it is all to easy to imagine a catastrophe, such as a landslide, wiping out most or all of the fragile vertebrate species.
Life as we know it is a glorious contingency.