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Multicellularity is a neat trick, but it’s not all that easy to pull off.
Before going multicellular, a few fundamental problems have to be
worked out. The development of a complex plant or animal always
starts from one single, fertilized cell made from the fusion of sperm and
egg. This zygote starts dividing like mad, producing the millions of cells needed to make a large organism. The strange beauty of this development is that somehow each cell “knows” what kind of tissue it is to
become part of—a slice of liver, the tip of your nose, a piece of your
heart, or a piece of your mind. How do they know?
Some scientists used to think (quite reasonably) that the instructions
were split up so that your toes got only the genes to make toes and your
nose had only nasal genes. Nope. It turns out that the entire genome,
the whole instruction manual, is present in every cell. This is why
embryonic stem cells have such great versatility. Each dividing cell of a
developing embryo has the potential to generate any body part. There
is some system—which hasn’t completely been worked out yet—for let-
ting cells know which kind of tissue they are to become. Without such a
master control system, multicellularity would not be possible.
This problem is solved through a multilevel genetic control system. It
is as if there is another genome within the genome of each cell that
somehow learns what kind of cell it is to be and activates certain genes
(calls them to action) and suppresses others (asks them to sit this one
out). This centralized control is quite elaborate, yet it is essential to
have something like this in place if multicellularity is going to work.
Difficult things do take time to evolve, but can that explain a 3-billion-
year gap?
Alternatively, we can ask what changed on Earth during this interval
that may have made the leap possible or even inevitable. One thing that
changed was the mixture of gases composing our atmosphere. Oxygen,
good old O2, which was apparently absent from Earth’s earliest air,
gradually increased in concentration until it became quite abundant,
second only to nitrogen. The most common explanation for the timing
of the multicellular leap is that it had to wait until the concentration of
atmospheric oxygen rose high enough to make efficient respiration pos-
sible. When you blow lightly on a flickering flame, it flares up.
Similarly, the more oxygen there is around, the more organic fuel life
can burn in the slow flames of metabolism. Perhaps the increased
energy source of an oxygen-rich atmosphere was needed to power the
larger bodies of the metazoa.
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Where did all this bountiful O2 come from? Life made it as a way to
avoid starvation. The history of oxygen on Earth is one of life coevolv-
ing with the planet.
L E T T H E M E A T L I G H T
When life first formed, finding a meal was not a problem since the early
Earth was loaded with organic molecules. But this paradisiacal situa-
tion, with tasty molecules everywhere for the taking, could not last. As
the food supply became depleted, a new long-term, reliable food source
was crucial for the continued survival of life on Earth.
Fortunately for us, before it was too late, life evolved the trick of
photosynthesis—using the energy of sunlight to make organic food.
This had to happen fast. Otherwise, when all the leftover crumbs were
gone, the party would have been over. Life would have perished, unless
it found a way to mooch off the geothermal energy coming out of sub-
marine vents or other exotic sources. But Earth life is almost entirely
solar-driven. Had life not developed photosynthesis early on, our planet
would now be unrecognizable, and there might well be no one around
this part of the galaxy capable of recognizing anything.
Photosynthesis is so pervasive and essential to life on Earth that it is
not inaccurate to describe the biosphere, as did Russian biogeochemist
Vladimir Vernadsky in 1926, as a continuous, thin film enveloping the
planet, within which sunlight drives matter through incessant transfor-
mations. In other words, life is something the Sun does to Earth. Earth
life is the way (or at least one way) that our star has found to express
its biological potential. We are the life of the Sun.
Photosynthesizing microbes began ripping the H out of H2O, com-
bining it with carbon to make organic food, and spitting out O2 as
waste. Even so, at first oxygen did not build up in the air. Everybody
loves oxygen, and many other elements were in line for its favors. The
young Earth had lots of iron, which is an oxygen hog if ever there was
one. For hundreds of millions of years, most of the photosynthetically
released oxygen went into oxidizing Earth’s iron, which was constantly
showing up in volcanic flows fresh from the interior, always demanding
oxygen. There was also methane (CH4) from volcanoes, methane from
bacteria, ammonia (NH3) from lightning storms, and other hydrogen-
rich gases, all composed of elements that eagerly forgot their other
dance partners the moment oxygen entered the room.
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There was always more than enough water to go around, and plenty
of sunlight. Life went on using both and leaving oxygen behind.
Eventually, the world’s thirst for oxygen was sated, or at least the
demand leveled off. Finally, free oxygen started ever so slowly to build
up in the atmosphere.
Hallelujah!
Except that this was a terrible disaster.
Oxygen is dangerous. Because of the promiscuous reactivity of oxy-
gen, no organic molecule is safe. Rust never sleeps. Have a little fire,
scarecrow. Oxygen was poisonous to organic life. The buildup of oxy-
gen was Earth’s first global environmental crisis, and life brought it on
itself. At first, it seemed as though the careless photosynthesizing
microbes had really screwed themselves, with their shameless oxygen
emissions causing global change that threatened their own extinction.
L I F E E X P L O D E S
Catastrophes, viewed from a different angle, are often opportunities.
It’s true that oxygen reacts ferociously with organic molecules. It’s also
true that these reactions release a lot of energy. Uncontrolled, this
energy will burn you up. But life, turning adversity into advantage,
found a way to harness fire: respiration, which uses controlled oxida-
tion to constantly charge its batteries. Aerobic life was born.
When respiration started, oxygen was still only a minor trace gas in
our atmosphere. Over billions of years, it gradually built up until it
reached its present level about 1 billion years ago. Since then, it has had
minor ups and downs but has not strayed too far from 20 percent of
the air we breathe.
The buildup of oxygen in Earth’s atmosphere had another tremen-
dously fortunate and enabling side effect: it made the ozone layer. Once
there was enough oxygen in the air, ultraviole
t light started splitting O2
and recombining it in various ways in the upper atmosphere. A by-
product was the production of O3 (ozone). Ozone happens to absorb
the same wavelengths of solar ultraviolet light that are fatal to our kind
of life because they destroy complex organic molecules. Before ozone,
life mostly hid under the surface layers of the ocean, where water
absorbed harmful UV. Now it was safe to colonize the land.
Like an athlete trained at high altitude coming down off the moun-
tain, life reveled in the ever richer air and revved up its metabolic
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engines. But if more O2 allowed the possibility of bigger living crea-
tures, you would never have known it for most of Earth’s history.
Respiration was around for billions of years before multicellularity.
Life did not get gradually larger as the oxygen level rose, but stayed
unicellular. Maybe this was because it could not make the move until a
certain threshold was crossed—a minimum oxygen richness for big life.
When multicellularity did arrive, however, its entrance was not sub-
tle. After waiting for so long, it burst onto the scene dramatically. Six
hundred million years ago, animals materialized, and soon they were
everywhere, appearing in a multitude of forms. We call this event the
Cambrian explosion.
It was as if some repressed creative force in nature was finally set
free. The evolutionary creation of major animal body types is not some-
thing that began in the Cambrian and has continued to the present day.
Rather, these templates seem to have mostly formed all at once. Species
come and go, riffing endlessly on the grand structural themes that were
all established at the time of the Cambrian explosion.
G E T T I N G S M A R T
Once we became multicellular, entirely new possibilities for organiza-
tion and specialization opened to us. Legions of cells could now be
called upon to dedicate their lives to specific structures and tasks.
Skeletal, circulatory, and digestive systems provided the infrastructure
for large bodies. Muscles and limbs sprouted for swimming, running,
climbing, and flying.
It doesn’t do any good to have such nifty toys if you can’t control
them, though. A nervous system was needed. The ability to sense the
environment and respond to it would be a definite plus. From such
humble needs were born refined senses and mechanisms for sending
signals throughout the body. Now you’re receiving information and
coordinating reactions and movements, so you need some kind of cen-
tral processing site. If you only had a brain.
Once you’ve got that, then you’ve got the rudiments of a cognitive
system. You’re on your way now, kid. You’ve got a leg up if you can
respond to your environment in flexible ways. This creates a pull for
more complex nervous systems with increasing power to sense, manip-
ulate, navigate, anticipate, and remember the world.
Then, 600 million years after the Cambrian explosion, something
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new happened. Matter woke up in human form. Was it a gradual focus-
ing or a sharp blink of self-recognition? We don’t remember. I doubt
we’re fully awake now. Intelligence, consciousness, self-awareness, the
divine spark of Jah—call it what you will.* We don’t know exactly
what crucial ingredient was added, or what threshold value was sur-
passed. One species started talking, building tools, and creating new
societies, not just out of instinct but through culture, ritual, and design.
A number of changes happened quickly as our hominid ancestors
sprinted into the new, big-head niche. We developed opposable thumbs
for grasping. An upright stance freed those hands to make tools and
throw spears. Language gave us the ability to communicate complex
ideas within groups. A rapid increase in brain size, doubling in about a
million years, facilitated all of the above, but made childbirth difficult
and dangerous. Evolution responded by pushing birth back earlier in
development, so that our expanding heads could make it through the
birth canal. As a result, humans, compared to most other animals, are
born unformed and helpless. This required prolonged and attentive
infant care, which increased social cohesion.
Recall that previous great leaps of evolution involved new associa-
tions between preexisting simpler organisms. Similarly, the origin of
human beings is inseparable from the origin of human societies. The
beginnings of language, the rapid growth in neural capacity, the forma-
tion of social groups with division of labor and the ability to plan and
learn collectively—all seem so tightly linked that it may be meaningless
to ask what caused what. These abilities seem to have bootstrapped one
another into existence. With language came the advent of a powerful
new form of heredity. We are cultural animals, and we pass on informa-
tion through word of mouth and artifacts: artworks, songs, rituals,
rhythms, stories, and now books, films, and disc drives. Our minds
expanded beyond our bodies and our thoughts came to survive the
death of individuals. A new kind of multi-organismic structure—the
society—was born.
Less than one hundred thousand years ago a small band of us, possibly
no more than fifty brave souls, left our native Africa and began spreading
*I’m not saying that other animals don’t have some degree of it. Some elephants can play the marimba pretty darn well.
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around the Earth.* These pioneers are your ancestors, unless you are of
pure African descent, in which case their less restless cousins are your
ancestors. Perhaps only fifty thousand years ago, we started using syntac-
tical, symbolic language, which allowed us to communicate abstract con-
cepts and poetry and planted the seed for the language of mathematics.
The more we can understand the tangled causal relationships that cat-
alyzed the jump to human awareness, the more informed will be our spec-
ulations on the likelihood of similar evolutionary events on other worlds.
T H E P S Y C H O Z O I C A G E
However it happened, it is clear that in just a few million years, in
barely the blink of a cosmic eye, one lineage of primates went through
an intense metamorphosis, and Earth acquired thought and self-
awareness. Several scientists and philosophers have recognized this as a
profound moment of transformation in Earth history. Pierre Teilhard
de Chardin, the Jesuit paleontologist/philosopher described it in 1955
as the beginning of a new geological age, the “psychozoic era.” He
described the web of interacting thoughts, communications, and arti-
facts rapidly covering the Earth as a new terrestrial sphere, the noo-
sphere (new-oh-sphere), which emerged out of the biosphere as the bio-
sphere had emerged out of the rocky lithosphere.
Although I cannot follow Tielhard all the way to his Christian con-
clusions, I find his vision of the human place in Cosmic Evolution to be
prescient and inspiring. I am with him when he says, “With hominiza-
tion, in spite of the insignificance of the anatomical leap, we have the
beginning of a new age. The earth gets a new skin. Better still, it finds
its soul.”
Am I giving us too much credit here, going on about the arrival of
humanity as though we were the second coming of sliced bread? Does
this view of planetary history place humans at the apex of creation?
Isn’t this picture of evolution a hopelessly self-serving glorification of
the human race?
Not necessarily. Read on, and you will find that I do not see us as the
apex of evolution. I believe that humans are the resident example of
something extremely significant on a cosmic scale. We may not even be
*Some scientists believe, on the basis of DNA studies, that it was less than fifty people.
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a particularly promising example. But that “something” is essential in
Cosmic Evolution. Without it, what’s the point? I believe we are part of
the very beginning, locally, of a phenomenon integral to the conscious
awakening of the universe. Whether we will be part of later stages
remains to be seen.
Conveniently, this realization gives us a new way to be human-
centered, even though science has robbed us of an Earth-centered cos-
mos and a purposeful human creation. Are we conflicted, hapless
humans really the vanguard of a new, conscious phase of Cosmic
Evolution? Deserve it or not, we are.
Lest our already dangerously bloated braincases become even more
swollen with this thought, keep in mind that of the species that ushered
in the age of multicellular life in the Cambrian explosion, all are long
gone and forgotten to everyone but curators of paleontology. But multi-
cellularity lives on. The phenomenon is more important than the
ephemeral particulars of who was first. Humanity does represent some-
thing the likes of which Earth has never seen, but the jury is still out on
our legacy and longevity as a species.
It seems as though, on principle, some people want to deny any evo-
lutionary importance to humans. I see this as a reaction to the historical
tendency to assume that humans were the purpose of creation, the cen-
ter of the universe and the pinnacle of life. We now know better than to
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