[mojosam]

> I don’t believe genetics ever claimed to provide a theory of why eyes grow where eyes grow.

That’s the whole point of developmental biology, to show how features of the human body form and develop based on gene expression, the timing of which during embryonic and fetal development itself is dictated by your genes.

If not your genes, what else would determine why you have eyes in about the same place in your head as every other human?

> The cells in your eyes have exactly the same DNA as the cells in your big toe, so developmental morphology cannot be explained with DNA alone.

Sure it can, because while every cell has essentially the same DNA, the expression of genes differs between cells, which is what causes cells to differentiate. And this differentiation also controls development; look up the Hox genes as an example.

[kevlened]

He's changed wild-type planarians to grow the heads of other species. It reverts after a few weeks, because the system has error-correcting mechanisms, but the DNA of these worms is unchanged.

He once compared tinkering with DNA as pulling out a soldering iron to fix a software bug.

In the case of morphology, DNA may not be the best level of abstraction. It's certainly possible, just as one can use chemistry for social problems, but for some problems, affecting cell-to-cell communication may be a more direct path.

[mmooss]

> If not your genes, what else would determine why you have eyes in about the same place in your head as every other human?

Theoretically, it could be second or much higher order effects that result from genes. It could be a combination of complex factors - the environment in the womb, nutrition, behavior by the mother, etc. - that eventually trace back to DNA.

Also, is it literally true that DNA is the only thing that's consistent (in these respects) between all generations of Homo sapiens?

[zmgsabst]

Your last paragraph is their point: genes are regulated to produce that effect. The genes themselves aren’t doing it, but eg diffusion of chemical signals to inactivate genes.

Morphology is determined by the combination of genes, chemical signals, original cell machinery, and apparently electrical signals. But we never believed that genes determined morphology alone, eg, we know that chemical signals can cause anomalies.

[mmooss]

> Morphology is determined by the combination of genes, chemical signals, original cell machinery, and apparently electrical signals. But we never believed that genes determined morphology alone, eg, we know that chemical signals can cause anomalies.

For the consistent parts - eyes may be different colors but are overwhelmingly consistent - what else could be the ultimate cause but DNA? For example, if those chemical signals, cell machinary, and electrical signals produce the same results billions of times over 200,000 years, then they must function the same overall. How does that happen if the chemical signals, cell machinary, and electrical signals aren't determined, even if indirectly, by DNA?

[zmgsabst]

An example of the contrary:

Your eyes would be misplaced if the process from cell clump to mat to tubule failed due to chemical signaling failure, but the whole embryo tends to be spontaneously aborted when gestation fails so catastrophically.

And despite genitalia being roughly one of two forms and similarly positioned, chemical signals can disrupt their formation.

> How does that happen if the chemical signals, cell machinary, and electrical signals aren't determined, even if indirectly, by DNA?

They don’t produce the same results with perfect accuracy — 75% of pregnancies are spontaneously aborted, at least in part due to developmental failures.

But the problem with this argument is simple: you have a human cell everywhere you have human DNA, so those correlations with DNA are also correlations with cellular machinery and with particular chemical signals from the mother. There was no point in those 200,000 years where DNA operated independently of those other mechanisms — we can only say the system as a whole reliably creates those features.

[mmooss]

Interesting points, especially about the challenge of correlation. I guess we could remove DNA and see what happens ...

Somehow the machinary is passed down: Do we know of another mechanism besides DNA that is self-perpetuating? Is there any living creature without it? Prokaryotes (bacteria) even have DNA.

Or is there a way to do it without self-perpetuating mechanisms? Is that logically possible? Some machinary might be perpetuated by other machinary, e.g. the chemical might recreate the electrical, meaning it's not self-perpetuating. But that's not different than DNA: DNA itself isn't the machinary, but its self-perpetuation is what recreates other parts.

I suppose some parts of the environment are consistent, such as sunlight, air, water, and heat, but the environmental stimuli must trigger something that is already there.

[zmgsabst]

> I guess we could remove DNA and see what happens ...

If I have a stool with three legs, and remove one leg causing it to fall, can I conclude that removed leg is what made it stand?

You’re making the same mistake as before in reverse: DNA would do nothing without a host cell or chemical signals, either.

> Somehow the machinary is passed down: Do we know of another mechanism besides DNA that is self-perpetuating?

The system as a whole is self-perpetuating, but DNA is not self-perpetuating: without a host cell and without ambient chemical signals, it cannot propagate. That’s in contrast to ribozymes which can be self-catalyzing RNA, ie, truly self-propagating chemicals.

In the RNA world hypothesis, such self-catalyzation was the origin of life; and by the time DNA evolved, it did so within a running biological system and as merely one component of cellular replication.

As a whole the system of chemical signals, DNA, and cellular machinery propagates; but just like our stool example, removing any of the factors causes that to fail.

[mmooss]

The DNA removal comment was as joke; sorry if that wasn't clear.

No system is self-perpetuating, per the Second Law; all need other inputs. What makes the machinary yield the ~same results ~every time is DNA.

> In the RNA world hypothesis, such self-catalyzation was the origin of life; and by the time DNA evolved, it did so within a running biological system and as merely one component of cellular replication.

Is there evidence of that? Afaik the earliest evidence is prokaryotes ~~3.5 billion years ago, and prokaryotes generally have DNA.