If we are committed to the idea that there is no design in nature, there is no comprehensive answer to this question:
So what are the timers that keep things trucking along at the right rate for any given organism, ensuring that it grows to the proper size and with all its parts in place? It’s a question that James Briscoe, a developmental biologist at the Francis Crick Institute in London, would love to answer. Take one lone example among many: motor neurons, the nerves that make muscles contract. These develop from precursor cells over a few days in mice but a week or two in humans — and the same thing happens when the cells are grown in a dish. “We can look at this carefully and demonstrate it’s the same genetic process, the same gene activities, the same mechanisms involved, and it’s just running slower in humans than in mouse embryos,” Briscoe says. “We’re trying to tackle that problem.”
Eryn Brown “How does the embryo make all its parts at just the right moments?” at Knowable Magazine
The research group can iterate in the greatest detail all the examples of the perfect timing throughout nature, via exquisitely designed controls, but dogma requires them to insist that it is all just an accident. A question arises: Will that requirement become a conceptual handicap?