to all living cells on Earth? How did organics beget organisms? This, not
some shadowy ape-man, is the real “missing link” in evolution.
The link may have been found in the form of remarkable chemicals
called ribozymes, the discovery of which bagged a Nobel Prize for
chemists Thomas Cech and Sidney Altman in 1989. Ribozymes are
molecules of RNA that, in addition to encoding information like other
nucleic acids (including DNA), can also behave like proteins. That is,
they can catalyze and orchestrate chemical reactions between other
organic molecules. This is huge, because in all life today that catalyzing
role is played by proteins. If a nucleic acid such as RNA can do it, there
could have been life without proteins.
Another way of putting it is that DNA, strictly speaking, cannot self-
replicate. It needs its dance partner, protein, to propagate itself. If mol-
ecules of RNA can somehow catalyze their own duplication, then they
would be true self-replicators.
A popular idea now is that life emerged through an early stage where
RNA was both the keeper of the genetic code and the catalytic enabler
of replication. At one point on its travels from chemistry to life, Earth
may have been an “RNA world,” awash in self-replicating ribozymes.
Is RNA world really the missing link between complex chemistry and
sophisticated cell? There are still a few loose rungs on the ladder of
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Cosmic Evolution. A description of RNA world is valuable as a “proof
of concept.” Like building the Kon-Tiki* and sailing it across the
Pacific, the RNA-world theory removes some of the historical mystery
by showing that the task could have been done with the tools at hand.
It bolsters the faith of those of us who think we can see an emerging
path winding from chemistry to life.
*A balsa wood replica of a prehistoric South American raft, which was sailed from Peru to Polynesia in 1947, in an attempt to prove that Polynesia was first settled from South America.
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Originally you were clay. From being mineral, you
became vegetable. From vegetable, you became ani-
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mal, and from animal, man. During these periods
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man did not know where he was going, but he was
being taken on a long journey nonetheless. And you
have to go through a hundred different worlds yet.
There are a thousand forms of mind.
—RUMI
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The difference between an amoeba and a human is
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one step.
—KARL POPPER
G O I N G C E L L U L A R
If we started out as self-replicating strands of naked RNA, we did not
stay that way for long. Soon the RNAs (or whatever the first genetic
stuff was) started assembling coteries of attendant molecules to help
them survive and reproduce. Catalysts, such as RNA, can selectively
surround themselves with other molecules, assembling some and
attracting others, choosing their company to control their immediate
chemical environments. Self-replicators that surrounded themselves
with the right molecules would come to dominate the mix. The “right
molecules” would be those that helped them to replicate.
A class of organic compounds called lipids spontaneously form
spherical structures in water. These thin linear molecules are water-
loving (hydrophilic) on one end, and water-fearing (hydrophobic) on
the other.
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Put a bunch of them in water and they quickly form spheres, called
vesicles, by “circling the wagons,” forming a spherical wall against the
surrounding water, with the hydrophobic ends huddled together inside.
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These spontaneously forming structures are suspiciously similar to
the membranes surrounding the cells of modern organisms, which are
also dominated by lipids. Any primitive genetic material that could
make or gather lipids would soon find itself protected from the envi-
ronment by a surrounding vesicle. This is probably how the first cells
formed.
Becoming cells was a giant step in our evolution. Perhaps this was
the moment of the actual origin of life. Before this, life (if you could call
it that) was a sea of chemicals with no clear boundary between one
individual and the next, no separation between living creatures and
their surroundings, and thus no real organisms.*
With the advent of membranes to divide interior from exterior, life
gained the potential to regulate its internal environments. Here on
Earth, life is chemical. The stability of those chemicals, and the rates of
their reactions (our metabolism), require specific temperature, acidity,
and concentrations of many trace chemicals. Organisms that could con-
trol any of these aspects of their internal environment had a huge
advantage over others that could not.
This was the microscopic equivalent of learning to live in caves or to
build huts to survive changing climate. When cells evolved the means to
regulate their interiors, they were not helpless against the randomly
changing chemistry of their watery home. After this, the chemistry of
life was forever more immune to harmful or fatal changes in the
weather. The internal environments of cells became stabilized by vari-
ous chemical and physical feedbacks that evolved to increasing sophis-
tication. Competition for survival was no longer between naked self-
replicating molecules, but between evolving cells.
C O M E T O G E T H E R
However it happened, wherever it has happened, the birth of life is big
news in Cosmic Evolution, right up there with the formation of quarks,
atoms, molecules, stars, and planets. Smaller, previously assembled bits
of the universe had repeatedly banded together to make big, new
*On the other hand, maybe the first life-form was actually this undifferentiated ocean of organic replication, a global organism that lived before differentiating into cells.
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things. But something changed with life. Before this point, matter self-
assembled only by answering the call of various forces of attraction.
There was never any blueprint. The advent of heredity marked a huge
shift. The cosmos, having stumbled upon chemical memory, would
never be the same. It could now start learning from its mistakes. Life
evolves by throwing possible organisms into the world, seeing what
survives, and passing the successful designs on to the future. Life
remembers.
An average humanoid handful of times, the universe has undergone
major changes in the way it pulls itself together, differentiates, orga-
nizes, evolves, awakens, and reflects. Heredity was one of these giant
steps, allowing entirely new possibilities of change, facilitating a meta-
morphosis in the roles of matter and energy. I think that the advent of
conscious awareness* is probably the next change on this scale. But,
how did we get from there to here?
Once the twisted ladder of DNA was firmly planted in the primordial
ooze, it kept building on itself. Improving and diversifying its reproduc-
tive tricks, it built more complex cells, then multicellular bodies with
specialized organs, nervous systems, sensory structures, hair, scales,
feathers, and flowers. From DNA’s perspective, all Earth’s creatures are
stationary or mobile reproduction units, even those weird new brainy,
bipedal ones that make fire, print books, and build rockets. All this in
the service of the master molecules. We are DNA’s spaceships preparing
for launch and trying not to get ourselves killed.
There are many ways to describe the history of life on Earth. We
could think of it as a sequence of geological dioramas—you know,
Jurassic©, Cambrian, Rastafarian, Bohemian—with depictions of the
flora and fauna populating each. But the procession of species making
cameos in life’s rich pageant is largely irrelevant to the questions of life
on a cosmic scale. Chance seems to be behind so much evolutionary
innovation. The intricate, colorful details of life’s transmutations are
fascinating to study and beautiful to behold, but they are transient rip-
ples in the sinuous stream of evolution. The profundity is not in the
details but in the breakthroughs, the moments when the evolutionary
river spills its banks and surges in new directions.
The truly cosmic changes, in this sense, are those involving major
*Something at least as difficult to define as life. I prefer to define it as something humans have not yet fully attained, but I’ll get back to that.
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transformations in basic biochemistry, in energy source, in reproductive
and hereditary mechanisms, and in the level of complexity and organi-
zation of life that allowed new abilities to emerge.
The first of these transcendent changes was the formation of the cell.
Echoing the pattern of Cosmic Evolution, many of the key develop-
ments of cellular life on Earth involved the formation of larger assem-
blages from smaller, simpler subunits. The first cells were prokaryotes
(pro carry oats). They resembled bacteria today in that they possessed
no internal organization. These earliest cells were just rudimentary
watery sacs with all of life’s chemicals mingling together, a well-mixed
cocktail of nucleic acids, proteins, and other organic molecules.
The next important transition came when cells began to differentiate
internally, acquiring subunits that were specialized for certain functions.
All cells in our bodies, and those of all creatures except bacteria, are
eukaryotes (you carry oats). These more complex cells have their genetic
material separated into an internal nucleus, distinct from the rest of the
cell. Inside eukaryotic cells, various tiny subunits, or organelles, devel-
oped to play specific roles. Small units called ribosomes became the sites
where proteins were assembled from DNA instructions, with RNA act-
ing as intermediary messengers. Other specialized structures brought
new abilities. Organelles called chloroplasts and mitochondria took on
the functions of photosynthesis and oxygen respiration respectively.
How did these organelles come to inhabit every living eukaryotic cell
(every cell of every animal, plant, or fungus that ever lived)? These were
early instances of evolution by symbiosis, in which once-separate
organisms begin living in close association and then, somehow, merge
into larger unified organisms. The energy-transforming subunits of ani-
mal and plant cells (mitochondria and chloroplasts) have their own
evolutionary history. Their ancestors were once separate bacteria, living
independent lives. Somehow these little guys gave up their individuality
to become the energy-processing parts of the collective eukaryotic cell.
The joining of formerly separate organisms to form new superorgan-
isms with expanded abilities is a persistent theme in evolution. Much of
the evolution of complexity in Earth life comes from the symbiotic join-
ing together of simpler organisms.
Certain steps in evolution have a Borg-like quality. For those of you
who have just woken up from a decades-long coma or are for some
other reason unfamiliar with Star Trek, the Borg is a fearsome entity
that evolves by assimilating other species, incorporating their technol-
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ogy and culture into the Borg Collective. The price for becoming part
of the ever-growing perfection of the Borg is that you give up your indi-
viduality. On Earth, complex cells were created by the assimilation of
once separate, simpler life-forms whose abilities were added to those of
the collective. Resistance was futile. We are the Borg.
Symbiosis is a biological form of “enlightened self-interest.” Survival
of the fittest still applies, but often the most fit are those who can form
strategic alliances with others. Extreme cooperation creates a new iden-
tity born of the cooperative as a whole. A new kind of individual
emerges when the group starts to reproduce as a single entity.
In evolution, the continual, random meandering of forms and species
is largely fueled by competition. But the real progress, the major inno-
vations, often involve new forms of cooperation between formerly sep-
arate creatures.*
The next great leap forward in the coming together of life on Earth
occurred when large numbers of eukaryotic cells joined together to
form multicellular individuals, or metazoa (big life). Even when indi-
vidual cells grew much more sophisticated, they were still just cells.
Cells can’t do certain things. If you want to fly, see, grow skin and
bones, dance the merengue, or calculate triple integrals, you are going
to want to go multicellular.
On the face of it, it is mysterious that single-celled creatures, each
acting on its own imperative to survive and reproduce, would give up
their individuality to become part of a new, larger individual. The
advantages to you—the metazoans—are clear enough.† But what’s in it
for your cells? Why should they give up their sovereignty to join the
United Cells of You? From an evolutionary perspective, the most pre-
cious thing that any creature has is the ability to reproduce. Somehow,
in the metazoan contract, individual cells ceded the power of reproduc-
tion to the centralized cells residing in specialized reproductive organs.
What the other cells got in return was the chance to be in on the
ground floor of something new and really big, with opportunities for
advancement and innovative ways to survive. Multicellularity allowed
*Some biologists and philosophers are fond of saying that there is no progress in evolution. On a certain level, this is obviously true. On a more profound and interesting level, it is obviously wrong.
†If there are any microbes reading this book, I apologize for my presumptuousness and i
nsensitivity.
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whole new levels of differentiation and organization. Multitudes of spe-
cialized cells developed into novel tissues and organs. Animals, plants,
and fungi: we are all consortia of huge numbers of eukaryotic cells,
which are themselves forged from ancient bacterial alliances.
W H Y T H E W A I T ?
Did life need to be multicellular? My perspective is warped, since I’m
writing this book with fingers for typing, eyes for reading, and a rudi-
mentary brain for musing. I like being multicellular, and I’m quite
fond of people, cats, redwoods, sea turtles, and prickly pear cacti. From
where I sit, it’s all the freaky multicellulars that make the beauty of the
world.
If microbes could talk, maybe they’d tell a different story. But they
can’t. Then again, in a sense, microbes can talk—in the same sense that
human beings can build space stations and chunnels. Overbudget and
behind schedule? No. What I mean is that an individual human can’t
build a space station, but complex organized groups of us can.
Microbes manage to talk by getting together to make us, by becoming
multicellular.
When we look over the history of life on Earth, a gnarly question
leaps out at us: If multicellularity is so cool, why did it take so long?
There is a gap of roughly 3 BILLION YEARS between the time when
cells were born and the time when they figured out how to join together
in large numbers to make animals. Three billion years is a long wait by
any standard—human, geological, even cosmological. It is more than
half the age of Earth and probably about a quarter the age of the uni-
verse. A 3-billion-year gap is no temporal chump change.
I’ll admit I find this disturbing. If life always self-organizes into more
complex entities, why did it get stuck? What kept us for so long at the
stage where individual cells were the greatest show on Earth? We do
not know the answer, but we have no shortage of explanations. Maybe
cells first had to develop some special capabilities that took a long time
to evolve. Or maybe this leap required changes in Earth’s environment
that happened very slowly.*
*Note that this is not really an either/or situation: changes in the physical state of Earth and changes in biological evolution are not independent. Extreme proponents of the Gaia hypothesis believe that no separation at all exists between the two.
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