…about evolution, and you’re not just saying you are so you don’t appear to be some kind of flub-brained creationist, Darryl Cunningham has put together a great comic strip detailing a very general view of the principles behind evolution.
I disagree with the section on “you have to think of DNA as a vast chemical information database,” because that implies a conclusion that something was out there to read that data. Under current theory, there was a period prior to cellular life in which self-replicating chemicals dominated the planet – RNA (ribo-nucleic acids). Very simple, able to generate new copies of themselves (not because they’re smart, it’s just what they do, just like battery acid “knows” how to chew through flesh). DNA, a very similar chemical, is transcribed by forms of RNA, and it’s not so hard to posit that DNA also occurred in this soup of RNA, basically parasitising the RNA world.
Eventually, when conditions were right to generate lipid “shells,” DNA and RNA got trapped inside those shells and became the first genome. I think the current best explanation for this is waves against rock and clay – in a sea choked with organic chemistry, imagine the most polluted environment you can think of, a scummy sea somewhere off industrial New Jersey or perhaps Lake Erie in the 1970s, waves beating that phlegm against the shore and making foam. That foam, in this case, was a combination of lipids and a few small proteins, cast-offs from the RNA/DNA replication race. The bubbles that the foam formed, the really small ones, trapped small pockets of RNA/DNA countless kazillions of times inside lipid shells until one combination actually was able to continue operation inside the shell. That shell was really malleable, and had a lot of surface tension, so it stayed bound tight to the contents while remaining flexible – and some things could pass through it. Again, not intentional, it’s just what lipids do. They break when certain conditions arise, and they let certain things pass through them.
So there’s that bubble – filled with the soup from around it, with actively-replicating RNA/DNA in it, and as the materials inside it get used up and the cast-off unusable stuff gets concentrated, another principle of chemistry kicks in.
Osmotic pressure is that experiment that most of us (those who didn’t go to a religious school or were
abused home-schooled) did right around eighth grade – separate two parts of a water tank with a semi-permeable membrane that doesn’t allow salt or some other chemical to pass, and then pour a shit-ton of that chemical into one side. The water can pass freely, so the volumes to either side stay the same, but one side builds up a huge concentration of the chemical – and because the molecules of that chemical have a tendency to spread out into an even distribution, it creates pressure. So the membrane bulges out.
In the case of our little bubble, the chemicals causing the osmotic pressure are able to pass through the lipid barrier, either directly through the lipids, or through holes in the small proteins caught in the shell. (Proteins are just basically long chains of amino acids – they form tangled strings that each have specific shapes, because there’s only so many ways those aminos can fit together – so now imagine one of these chains tangled up in the form of a lowercase letter “e”, and caught in the lipid shell. Stuff that is small enough can fit through the hole in the “e”, making that protein a teensy little gateway for crap to go back and forth between the outside and the inside.)
So the DNA/RNA inside the shell can keep doing what it does, replicating, because although the lipid/protein bubble it’s stuck in does slow it down some, it doesn’t prevent new materials from coming in – and it lets byproducts out.
Eventually, though, the replicating RNA/DNA builds up enough of itself inside that it can’t fit easily – and the little bubble has to stretch, eventually splitting off little sections of itself to alleviate the pressure. Probably most of these just burst, spilling all their crap back into the soup. But a couple – at least one – somehow has the right byproducts of its DNA/RNA to make the shell flexible enough (remember, we’re talking chemistry – a lot of the stuff we’re dealing with is really slimy and slippery, and can have viscosity similar to motor oil or tar). So this one bubble with a bunch of DNA/RNA in it is spewing out enough byproducts inside that it can suck up stuff from the outside soup, replicate its nucleic acids, and rather than burst, its shell is of the right consistency to pinch off rather than pop, like bubbles of oil in water.
Mainly because that’s what it is, it’s a weird kind of oil bubble in water.
What that is, is a cell. I won’t pretend that this is what happened, because I don’t know – but it is a pretty good description of how DNA got its start inside a cell. And to bring it back to the original point above, let’s say that cell has a generation time of sixty minutes (e.coli is 20, but it’s a lot more complicated and has had time to fine-tune to its surroundings). It’ll have 8,766 generations in one year.
In a million years, it’ll get 8,766,000,000 generations. If each of those results in a doubling (it won’t, because some of the cells will be destroyed, but for the rough purpose of this discussion we’ll use it), that’s 2 to the 8,766,000,000 individual cells – each one subject to error in replication. Errors in such a haphazard collection of crap will be quite common, and more often than not fatal.
But some will be advantageous.
In case you’re wondering, another word for error in this context, is mutation. And over the course of 4,500,000,000 years (in the world of the cell described above – that would be 39,447,000,000,000 generations), there’s a lot of room for change.
So the accumulation of change to the tiny little bucket of DNA/RNA in that early, primitive cell results in the DNA we know today – where cell lineages have found survival tactics wildly different from one another. Just as one person might make a living as a nuclear physicist while his or her sibling might be a baker, cells since that early age have found an enormous number of different paths to survival and propogation. Some took on an extra lipid layer. Some bound together in great colonies and use chemical emissions to keep each other aware of the status of the colony (these are called biofilms).
Some went a few steps further, and started to specialize. Certain cells discovered that working in cooperation enabled them to focus on a symbiotic relationship – where some cells performed only certain functions. Eventually, these cells got so accustomed to living in symbiosis that they couldn’t survive without their partners – and these became the first true multicellular organisms.
The rest, as they say, is history. But DNA isn’t a database – it’s more like a cookbook. A really messy one, that gets scribbled out onto pages in crayon every time it gets transcribed. Each gene is a recipe for a protein (and some of the DNA, it is currently thought, is also a form of instruction). There’s also a lot of crap built up in our genome – the term “junk DNA” is used for this – which are broken genes, things left over from generations ages and ages ago that no longer has any function today. But because the crayon-scribbler is blind and doesn’t know what’s good and what’s not, it copies all of it, junk and all, and makes mistakes every time it does. (As an example, humans accumulate approximately 150 errors with every transcription – each of us is a mutant. We each have about 150 mutations in our genes relative to the genes that were donated by our parents.)
That’s a really long description there – which is why, for the purposes of a comic strip, using “database” is appropriate. But lest it become something that a proponent of Intelligent Design (a.k.a., and IDiot) tries to harness dishonestly as “evidence”, this description needed to be laid out.
So, again, go read the cartoon. It’s really great.