Earth is 4.55 billion years old, and for the first few hundred million
years, enormous impacts regularly melted the planet all the way through.
Any life that had formed during this violent time might have been eradicated.
Even after Earth had reached its current size and oceans had begun to form,
million-ton rocks continued to fall out of the sky every few million years.
If life existed on Earth when these impacts occurred, it might have survived
in hidden refuges, perhaps in the chambers of undersea volcanoes. But it
might also have become extinct. The last storm of titanic impacts occurred
3.9 billion years ago; 50 million years later, life was well established,
and 350 million years later sophisticated microbes were definitely alive.
How could such a complex genetic system have evolved so quickly? The
biologists who forged the modern synthesis of evolution mainly studied
how minor genetic changes -- a switch of A to G at one position in a gene,
for instance -- could add up to big evolutionary results. But it turns out
that there's another important ingredient to evolution: the accidental
duplication of entire genes.
Gene duplications occur at about the same rate as single-base mutations.
Once a new copy of a gene appears, it may end up with one of several fates.
It may make more of the protein the original gene made, which may raise the
organism's fitness. The protein may be essential for processing food, for
example, and making more copies of the protein lets an organism eat more
efficiently. In that case, natural selection will hold on to the duplicated
gene in a form much like the original.
But the extra gene may instead be superfluous. In these cases, a
mutation that strikes the new copy won't affect the fitness of the
organism that carries it, because the original gene is still doing its
job. Most of the time a mutation to a duplicated gene will simply render
it useless. Our DNA brims with these genetic ghosts, known as pseudogenes.
But sometimes mutations can transform a duplicated gene so that it makes
new proteins that can do new jobs.
The genomes of bacteria, archaea, and eukaryotes all contain hundreds
of duplicated genes, which can be grouped into families in much the way
species are grouped together. And in both cases, the grouping reflects a
common descent. Gene families are the work of many rounds of gene
duplication, reaching back to the earliest stages of life. Genes did not
simply mutate on the early Earth: they multiplied.