remained hot, ring molecules of sulfur atoms in the atmosphere would block
out solar ultraviolet radiation. Otherwise, the first living cells would have siz-
zled in the deadly rays of the Sun. However, an ultraviolet shield might not
have been necessary in the primordial atmosphere, because some primitive
bacteria appear to tolerate high levels of ultraviolet radiation.
Evidence that life began quite early in Earth history when the planet was
steaming hot exists today as archaebacteria, or simply archaea. They range
more widely than previously believed, and many parts of the ocean are teem-
ing with them. A third of the biomass of picoplankton (the tiniest plankton)
in Antarctic waters were archaea. Such abundance could mean that archaea
play an important role in the global ecology and might have a major influence
on the chemistry of the ocean.
The first living organisms were extremely small noncellular blobs of pro-
toplasm. The self-duplicating organisms fed on a rich broth of organic mole-
cules generated in the primordial sea. Such a nutritional abundance set off a
rapid chain reaction, resulting in phenomenal growth. The organisms drifted
freely in the ocean currents and dispersed to all parts of the world. Although
Figure 14Hot carbonated
springwater undercuts
bedded travertine deposits
at Yellowstone National
Park,Wyoming.
(Photo by K. E. Barger,
courtesy USGS)PLANET EARTH