6 Life’s Origins
To the Blazing Hot North Pole
On a sunny day in late July, I am off to North Pole to examine these vestiges [of
ancient life]. Heat and dust permeate the cab as our Land Rover rattles over the rutted
dirt track. There are flies everywhere. This North Pole, you see, lies in northwestern
Australia—its name, with characteristic Aussie humor, marking one of the hottest
places on earth.
—Andrew Knoll, Life on a Young PlanetHow did life on earth begin? This is one of the most fascinating and controversial top-
ics in science. Unlike many of the other areas of life’s history that we discuss in the remaining
chapters, we have only a few fossils to guide us. The oldest fossils that are clearly formed
by living things are microscopic fossils of cyanobacteria (formerly mislabeled “blue-green
algae,” but they are not true algae) from 3.5-billion-year-old rocks of the Warrawoona Group
near North Pole in Western Australia. In addition to these microfossils, there are layered
domed structures known as stromatolites (fig. 6.1) that are produced by cyanobacterial mats
(which are still producing such structures today) from the Warrawoona Group (fig. 6.2). Just
slightly younger are numerous even better microfossils and stromatolites from rocks dated at
3.4 billion years from the Fig Tree Group of South Africa. As this book went to press, possible
stromatolites dated at 3.8 billion years old were reported from Greenland, which would be
the oldest fossils known if they are indeed stromatolites.
All of these microfossils are simple filaments with a number of cells in a row, many with
distinctive structures that are virtually indistinguishable from their modern counterparts. So
we know for sure that bacteria and other simple prokaryotic (lacking a discrete nucleus) cells
were present 3.4–3.5 billion years ago, but so far we have not found any older fossils. This
is not surprising, given that there are very few places on earth with rocks any older than
3.5 billion years, and most rock that old has already been heavily deformed, cooked by meta-
morphism, or otherwise so altered that there is little chance of any fossils surviving. There
are rocks from the Isua Supracrustals in west Greenland that have distinctive organic mol-
ecules in them that are unique to living systems, suggesting that life was present 3.8 billion
years ago (Schidlowski et al. 1979; Mojzsis et al. 1996), and these are the rocks that also yield
the possible stromatolites as well (Nutman et al. 2016).
Certainly, it is likely that life was already established by 3.8 billion years ago. Whether it
was possible for life to get a foothold on earth before then is still debated. Before 3.9 billion
years ago, the earth was still heavily bombarded by leftover debris from the formation of the
solar system, which probably vaporized the oceans of the earth many times. (We know this
because most of the impact craters on the moon, which must have suffered through the same
bombardment, date from 3.9 billion years and older.) Scientists call this time the period of
“impact frustration.” Most geologists did not think the earth could have cooled enough for