To cast an even more interesting light on it, one of the possible—and credible—origin-of-life theories is that life came from outer space. Yes... life may be alien. The theory is more accepted than the idea that God created life out of thin air, at least in standard scientific circles.
One of the reasons this theory is considered credible is that DNA is an incredibly sophisticated, highly evolved and optimized molecule. It's so good at what it does that it just doesn't seem as though there was enough time after the molten fireball of earth cooled off enough to support life for it to have evolved to the state we find it in in the earliest fossil records... about 3.4 billion years ago. The question of how life evolved at all in the hostile environment of early earth is still actively puzzling scientists.
The earliest fossils are recognizably similar to organisms still around today, which just about guarantees they shared a version of the same DNA we find in ourselves. DNA, it turns out, is so very, very good at what it does that many of the structures it uses to create specific proteins, molecules, and body structures are preserved as analogs to one another in creatures that diverged five hundred million years ago or more.
Bodies change... niches change... but although in some ways DNA changes a lot, in other ways, it's actually a very, very conservative molecule. It rarely, if ever, re-invents if what it already has can be reconfigured to work in a new way. Organisms (we can think of DNA, in a broad sense, as an evolving organism) this highly developed, consistent, and durable probably took hundreds of millions of years to reach that level of sophistication; and this is, to be sure, rather unlikely to have taken place at an accelerated pace under the hostile conditions provided on the early earth.
In a certain sense, man himself is a collection of microbes, since each cell—fully integrated and cooperative though they may be—functions as an individual organism. We're reminded of this in unpleasant ways when we get cancer—this, after all, is the ultimate example of just how independent and, in fact, malevolently autonomous cells can still be under the right circumstances. White blood cells (macrophages) wander around acting on behalf of our defensive systems; cancerous cells act something like that, only when they crawl between tissues, they are looking for places to metastasize—settle down, set up house, and... eventually... kill us, if we're unlucky.
So we're communities of microbes, hosting larger communities of biota. It's estimated there may be as many as 100 trillion "foreign" microbes in the average human being. Multiply that by a planet of 6 billion people... we're up to six billion trillion microbes, and that's just the ones resident in us humans. In contrast, consider that there are probably about ten trillion of our own cells in a human body. The microbes we carry outnumber us by about 10 to 1... or 100 trillion to one, if you want to count our conscious self as one, and our microbes as being on the other side of that fence.
We live, furthermore, in a sea of microbes, one so dense as to give people with OCD the willies.
The point of this excursion is to explain that we depend on our microbial population for survival. We don't just have the microbes; we need them, and the vast majority of them have been evolving in conjunction with animals... first arthropods, then amphibians and fish, then reptiles, and mammals, until finally they ended up here with us. Each microbe we host is exquisitely tuned to living on or in our body.
It isn't just this way for humans. All of life on earth is ultimately arranged this way; everything depends on microbes to function.
We'll take that up in the next post.
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