38 LIFE FROM AN RNA WORLD
are larger distinct features of the Earth. But the well-known
supercontinets „„„pangaea, Gondawana, and Laurasia are still
too young. We must go back one billion years ago (1 Gya, a
gigayear ago, 10 9 or 1, 000 000 000 years ago) to Rodina, the
first supercontinent and the mother of all surviving continents.
The time required to break up Rodina, reform the super-
continental Earth, from and break up Pangea, form and
break up Gondwana, and watch the continents sail to their
current sizes and positions-that is 1 Gya and a unit com-
meansurate to the history of life on Earth.
Judging from the satelite-borne Wilkinson Microvave
Anisotropy Probe’s data on the leftforever radiation from the Big
Bang, the bang occured 13,7 Gya, with an uncentainty of only
200 milion years. A few hundred million years later, the first
stars began to shine, and the Milky Way galaxy, our home
came into existence. In fact, the apparently oldest stars
our
galaxy have an age indistinguishable from that of the universe
itself, so the oldest parts of our galaxy were formed fairly early
(though this time estimate has much greater uncertainty). The
figure of 13,7 Gya is an important number because it supplies
an upoer limit on the age of everything. Nothing can be older
than the First Singularity of tge universe. Thus when it is sug-
gested seriously that life on Earth has extraterrestrial origins
(and it is suggested seriously), no matter what distant parts of
the universe life might have come from, it cannot be mire than
abiut threefold older than the Earth itself. This is an impor-
tant fact-whether or not you are entertained by the idea that
we are extraterrestrials.
We can be more precise about the age of the solar sysetm,
including Earth. By measuring the decay of uranium isotopes
to lead in meteorites we know that the rocky material of our
solar system congealed at 4.55 Gya (with an uncertainity of