Body Plans
Implication of Spines
I don't know how the process of life got started on the Earth. Creationists claimed that the set of interlocked processes that enable life was too complex to arise spontaneously. Therefore they claimed there must be a supernatural creator who started life going.
I've heard of a few plausible suggestions for how life began. First we processes that form loops. State A leads to State B which leads to State C which leads back to State A. The processes use chemicals in their environment. Such a loop might tend to suck up all the required chemicals in their environment and so there would be scarcity. I can imagine that some of the loops would be more efficient at sucking up resources. Such processes would evolve.
I've read of loops like that are possible with certain clays as a substrate to hold molecules together for the reactions. I've read that possibly the chemical reactions around deep sea hot water vents might provide an environment for such replicating loops. I don't know what the first replicators were but I think that they must have been fairly simple. A central idea of Darwinian Evolution is that the whole of the diversity of the biological world evolved from that simple progenitor loop.
These days DNA provides pretty strong and direct evidence for that hypothesis. All of life is based on DNA. If life had many different starting points then we would find some life forms that aren't based on DNA and we don't. But DNA isn't actually easy to see. Only in the last hundred years or so have even suspected it's existence. The body plans of animals provide evidence that is easier to see.
Consider mammals. They all have a front and a back and a left side and a right side. At the front is a head with eyes and a mouth. There are forelegs and hindlegs arranged in mirror symmetric pairs. There's an anus and a tail. All mammals from a tiny shrew to a huge whale share that same abstract plan. And there is pretty much a continuum in the variations among different species.
So one way of looking at different mammal species is that they are successful variations of the same abstract plan.
Mammals are part of a broad classification of animals called the chordates. These are animals that have a spine that develops from a notochord in the fetus. Plants don't have notochords. Nor do jellyfish.
Creatures with spines move in a directed way to get the food they need. This is a design pressure. The need to move to find food sort of forces the structure of a head with a mouth and eyes and limbs. The process of digestion sort of forces the design to have an anus at the back.
This design pressure on creatures is an aspect of evolution that is distinct from mutations or even the variations caused by sexual reproduction. We tend to think of fitness in terms of competition among individuals - ie peacocks have fancy tails because the ones that do attract more mates and leave more offspring. A mutation that produces fancier tails will tend to spread among peafowl. This is distinct from the design pressure that keeps all peafowl to the basic chordate body plan.
What do you think?
I present regular philosophy discussions in a virtual reality called Second Life.
I set a topic and people come as avatars and sit around a virtual table to discuss it.
Each week I write a short essay to set the topic.
I show a selection of them here.