In “From Blue Whales to Earthworms, a Common Mechanism Gives Shape to Living Beings” (ScienceDaily, (Oct. 13, 2011), we learn how the embryo gets organized, hourby hour, in two days:
Very specific genes, known as “Hox,” are involved in this process. Responsible for the formation of limbs and the spinal column, they have a remarkable characteristic. “Hox genes are situated one exactly after the other on the DNA strand, in four groups. First the neck, then the thorax, then the lumbar, and so on,” explains Duboule. “This unique arrangement inevitably had to play a role.”
The process is astonishingly simple. In the embryo’s first moments, the Hox genes are dormant, packaged like a spool of wound yarn on the DNA. When the time is right, the strand begins to unwind. When the embryo begins to form the upper levels, the genes encoding the formation of cervical vertebrae come off the spool and become activated. Then it is the thoracic vertebrae’s turn, and so on down to the tailbone. The DNA strand acts a bit like an old-fashioned computer punchcard, delivering specific instructions as it progressively goes through the machine.
“A new gene comes out of the spool every ninety minutes, which corresponds to the time needed for a new layer of the embryo to be built,” explains Duboule. “It takes two days for the strand to completely unwind; this is the same time that’s needed for all the layers of the embryo to be completed.”
This system is the first “mechanical” clock ever discovered in genetics. And it explains why the system is so remarkably precise.
And just think, it all happened by natural selection acting on random mutations, … Well, let’s just say it would be illegal to introduce, in American or British schools, any kind of doubt that that’s the origin of the mechanism.
The Hox clock is a demonstration of the extraordinary complexity of evolution. One notable property of the mechanism is its extreme stability, explains Duboule. “Circadian or menstrual clocks involve complex chemistry. They can thus adapt to changing contexts, but in a general sense are fairly imprecise. The mechanism that we have discovered must be infinitely more stable and precise. Even the smallest change would end up leading to the emergence of a new species.”
Ah yes, there’s the ritual kowtow to Darwin (“who changed everything, greater than Copernicus” – E.O. Wilson), although in this case, the PR writers daren’t be explicit.
In reality, “Even the smallest change would end up leading to the … ” death of the embryo.
The real lesson here is the importance of model. Embryo development must be precise, like a clock, and not randomly mutated due to the chances of chemistry.