>> The more I read about molecular biology, the more interesting it becomes.
>
> Really incredible stuff goes on. I'm never sure whether I'm more amazed
> that it works like this, or that we can figure out that it works like this.

Indeed. It's not surprising that people win Nobel prises for this kind 
of work.

Take a look at the paper I linked to. Just look at how much statistics 
went into merely checking whether human opioid receptors have changed at 
all, and if so whether it's by chance or due to selection pressure.

(The idea of the human brain containing receptors for opium has always 
bothered me. Opium is a foreign substance that doesn't belong there; why 
the hell would every single walking human have receptors for responding 
to it? The answer, of course, is that they aren't *opium* receptors at 
all! They're endorphin receptors. They respond to the endorphins that 
the human brain itself naturally produces. And yet, even the word 
"endorphin" means "endogenous morphine". How backwards can you get?)

>> Mutations that are fatal are vigorously eliminated by natural
>> selection. And indeed, you can find genes that have barely changed for
>> billions of years. These are the so-called "highly conserved sequences".
>
> I remember reading somewhere that there's a gene that controls how many
> fingers you have *and* something about the reproductive system, so any
> mutation in that gene tends to keep you from reproducing for entirely
> unrelated reasons. Hence the reason why everything from fish to bats to
> birds to people have five finger bones.

Actually I think you'll find it's that all tetrapods are descendants of 
a single fish ancestor, which just happened to have 5 digits. By now it 
would be far too difficult to change it.

Incidentally, you may have heard about the theory that embryos retrace 
their ancestry as they develop. For example, a human embryo initially 
looks not unlike some kind of bizarre fish. Similarly, baleen whales 
initially grow teeth before later reabsorbing them (since their 
ancestors had teeth, but they do not).

Actually this theory is wrong. Embryos do not /literally/ retrace their 
evolutionary history. What /is/ true is that alterations to the early 
parts of embryology tend to bugger up more things than do alterations to 
the later parts of embryology. Not as an absolute rule, just a general 
tendency. The net result is that typically animals change their final 
body plan by changing the later stages of their embryology rather than 
the early ones.

In short, I suspect that tetrapods all have 5 digits because there are 
now highly complex, well-developed and extensively inter-dependent 
systems of gene regulation for building 5 digits. You'd have to change a 
hell of a lot of stuff to make it, say, 6. If you just changed one 
chemical gradient, for example, you'd break so much stuff... it just 
wouldn't work.

If you look at things that aren't tetrapods, you find that 5 isn't so 
special.

>> Speaking of which, here's a thing: Every single living cell in the
>> human body (with a few exceptions) has the exact same genome.
>
> Every single *human* cell. About half (or more) of the cells in your
> body aren't human, tho.

Humans are eukaryotes. Eukaryote cells are apparently *way* bigger than 
bacterial cells. So it's not difficult for bacterial cells to outnumber 
human cells; they're smaller. And while we're on the subject, everybody 
thinks of bacteria as "those things that make you ill". But the vast 
majority of bacterial species have no effect on human health at all. And 
a large number of them are /beneficial/.

(For example, E. coli is well-known for making people ill. But it's a 
normal resident of the human body, and it even "communicates" with the 
human digestive system, telling it for example how much nutrients it 
needs. The human digestive system then absorbs all but that amount, 
leaving enough behind for E. coli (and friends) to stay fed. They are 
symbionts.)

Then of course, there's the theory (presented by Richard Dawkins as if 
it's an accepted scientific /fact/) that most of the organelles of the 
eukaryote cell are actually symbiotic bacteria which have become reduced 
almost to nothing, retaining only their key chemical processes that 
benefit the cell. If it were true, it would mean that basically the 
entire chemistry of visible life is possible due to bacteria.

> I'm not sure why you have a job in IT instead of a job in teaching.

I'm not sure why I have an honours degree in computer science, and yet 
my sister actually gets paid to write computer programs. o_O

Then again, she works in London, so...

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