Geology Reference
In-Depth Information
ory” may have contributed directly to the rise of atmospheric oxygen and the evolution of
the modern living world.
The Invention of Animals
Hothouse algal blooms, aided by phosphorus and other nutrients, undoubtedly contributed
to the sharp spikes in atmospheric oxygen. The clay mineral factory may have amplified
the effect. And so by about 650 million years ago, atmospheric oxygen had risen to near-
modern levels. Elevated oxygen, in turn, has been implicated in the rise of complex mul-
ticellular life, for only with such high levels of oxygen could organisms adopt the active,
energy-demanding lifestyles of jellyfish and worms. Indeed, the earliest known multicellu-
lar organisms appear in the fossil record about 630 million years ago, just after the second
global snowball glaciation.
TounderstandtheriseofanimallifeintheNeoproterozoicEra,wemustfirstlookfurther
back, more than a billion years back, to just before the boring billion. The sparse fossil
evidencepointstotheriseofawhollynewkindofsingle-celledlifeabouttwobillionyears
ago. Prior to that time, all cells seem to have led physically separate, if codependent, lives.
But about two billion years ago, according to a revolutionary idea first expounded by bio-
logist Lynn Margulis at the Amherst campus of the University of Massachusetts, one cell
swallowedanotherwhole.Ratherthandigesttheengorgedcell,thebiggercellco-optedthe
smaller one in a symbiotic relationship that forever transformed life on Earth.
Margulisisacreativepowerhouseandanintellectualomnivore.Herscientificcareerhas
been devoted to understanding how groups of organisms interact and coevolve; she sees
symbiotic relationships and the sharing of biological inventions as a pervasive theme in
life's history. Her ideas have ruffled more than a few feathers, in part because they deviate
from the more orthodox Darwinian view of evolution primarily by mutation and selection.
In spite of the controversies, Margulis's theory of endosymbiosis is compelling and almost
universally accepted today. Modern plants, animals, and fungi consist of cells with many
internal structures—mitochondria that act like tiny power plants, chloroplasts that harness
the Sun's energy in photosynthetic organisms, the cell nucleus that holds the genetic mo-
leculeDNA.Theseandother“organelles”incomplexcellshavetheirowncellmembranes
and, in some cases, their own DNA as well. Margulis proposed that each of these organ-
elles evolved from earlier, simpler cells that were engulfed and ultimately co-opted to per-
formspecificbiochemicaltasks.Accordingtoourbestguess,thattransitionbegantooccur
about two billion years ago and set the stage for much more complex, multicellular life.
Marguliscontinuestoseelife'sevolutionasdrivenbysymbiosisandthesharingoftraits
among disparate organisms—a view that she has taken well beyond endosymbiosis (and
that at times places her outside the mainstream). One of her recent crusades, beautifully
