How can earth and water produce a live frog?

Pauline asked:

How can earth and water produce a live frog?

Answer by Craig Skinner

Up to the late 19th century, some educated people believed that frogs could arise directly from earth and water, rats from mud, and flies from compost – so called spontaneous generation. But not many thought that, and very few do nowadays.

Instead we think earth and water produce frogs through the intermediary of parent frogs mating, egg laying, and hatching of tadpoles which become new frogs. Normally we think of the frogs as doing it using earth and water, rather than earth and water doing it using pre-existing frogs, but it’s a matter of perspective and your views about causality and agency.

So there’s your answer.

Of course, the question arises as to how the first frogs arose. Before 1859, there was no convincing natural explanation, and the popular view was that the first members of each species had been specially created by God. Then Darwin and Wallace suggested evolution by natural selection: starting with a single, simple, life form, some descendants show heritable variations, and variants most fitted to their environment leave more descendants, driving the gradual appearance of variety and complexity.

Not only is this a great explanation for the varied biosphere, it is (I think) the only possible natural explanation. But there again, although the idea is obvious once you’ve thought about it, it eluded minds as great as Aristotle, Kant or Hume, and it could be that another explanation currently eludes all of us.

A brief aside: of course, once intelligent life emerges, it could itself create life, e.g. Venter labs produced the first bacterial cell fully controlled by a synthetic genome in 2010. Or you and I, and everybody else, may exist in a vast computer simulation run by superior intelligences or even by 26th century humans.

The chronology of life on Earth is well-documented now. Frogwise, the branching into amphibians (later including frogs) and amniotes (later becoming reptiles, dinosaurs, birds and mammals including humans) occurred 340 million years, and frogs have been producing other frogs, without much thought, for 300 million years.

How did the first life-form arise on Earth (probably only one because all life forms share the same genetic code)? Nobody knows. The when is about 3.5 billion years ago, but there is debate as to where and how.

As to where, popular suggestions include warm ponds with sunlight as the energy source; deep sea vents with superheated water providing the energy; clay surfaces providing a structure for chemicals to organize on; and more way out ideas such as arrival in meteorites knocked off Mars (then life-bearing) or from outer space in cosmic dust.

As to how, there is debate as to whether bare replicators came first (probably RNA) and protective coats (cells) evolved later, or whether dividing globules (cells) came first, and were later invaded by replicators (RNA) that divided with the cells, and came to control cell function. Either way, RNA was largely superseded by DNA as the genetic material.

Finally, as to whether frogs can sometimes become handsome princes, I have an opinion on that as well, but will let you make up your own mind.

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