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.
6 comments:
Obviously any thoughts on what life would be like if only one thing long ago was changed is speculation, but if the development of planet earth was the same, then we'd still probably see life similar to what we see today.
There are certain "shapes" of life that are more beneficial. Modern whales share a similar "shape" to the marine reptiles of the Jurassic. That is no accident. The streamlined shape does extremely well in a marine environment. Likewise, bipedalism seems to be directly linked with higher intelligence. When two limbs are not used for locomotion, the possibilities for them are endless.
If somewhere out there there exists a planet similar to ours with an intelligent species, I think that species will be similar to us in shape at least. It will most likely have eyes facing the front, be bipedal, etc. It may not be mammalian, or fall into any category of life here on earth, but I think the shape at least will be similar.
Brian,
I think you are missing the point. The ancient marine reptiles and whales are both vertebrate tetrapods, so they have the same basic body plan. However, we have never seen crustaceans or insects fill in these niches, who have an entirely different body plan.
We also have to consider the fact that only a single lineage has ever lead to what we might call "human intelligence".
Lots of external factors have determined what evolved and why and what went extinct (meteors and volcanoes mostly).
Major environment change motivated the Cambrian explosion, as many individual ecosystems changed on our planet...was it due to natural temperature on the planet? Volcanoes? Meteors? A combination?
I think the question you are asking is if mammals would have made it here regardless. I think that the emergence of mammals without a major predator would always lead to higher intelligence beings.
For us to be here today, we needed the dinosaurs to have been wiped out, that is the only thing for sure.
Hey BEAJ,
Very few novel body plans evolved after the Cambrian. Of all the body plans that ever evolved, only one, so far as we know, was a vertebrate.
If things had been just a little different, perhaps if there was a catastrophe that wiped out an early and small population of Pikaia, then vertebrates would have most likely never evolved again and we would not be here.
Maybe there doesn't have to be any change. Don't underestimate the importance of sheer dumb luck in evolution. Had a small group of pikaia not found a mate, had they been washed ashore, we might not be sitting here.
Lucky genetic drift might be the sole reason we exist, rewind the tape, add a single flapping butterfly, and jellyfish might be the dominant species.
Hi Alcari,
If determinism is true, and I think it is, "rewinding the tape" with everything exactly the same would produce the same results. However, if you were to do something simple, like say, alter the number of cosmic rays that reached the earth by a tenth of one percent (which are known causes of mutations) then we may not have ever evolved and something like jellyfish, or even something we cannot imagine would be the dominant species. In fact, alter things a little in the beginning, and life as we know it may not evolve to anything we even remotely recognize!
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