We are a way for the cosmos to know itself.
n the beginning there was energy. 13.7 billion years ago our universe burst suddenly from a point infinitesimally small and unimaginably dense, and expanded explosively into the dimensions of space and time with which we are familiar (and possibly also ones we’ve yet to discover). The vast somethingness that surrounds and comprises us unfolded like origami from a miniscule speck of nothingness. From the start it was moving – vibrating, rotating, wobbling, churning, and whirling hastily outward.
As the energy cooled it changed into tiny particles and then into baryonic matter, namely hydrogen and helium. One of the fundamental features of this fledgling universe was soon revealed: matter attracts matter. The hydrogen and helium coalesced into ponderous clouds, which coalesced further into clusters of galaxies, and within the galaxies the first stars flickered to life. Matter transformed itself into sunlight, though there were not yet planets or grateful inhabitants to receive it. The stellar forges fused hydrogen and helium into heavier elements. The first stars burnt through their fuel wantonly as they traversed the widening cosmic expanse, and by an odd trick of nature the most massive ones ended their lives in dramatic explosions, dumping their precious cargo – oxygen, carbon, iron, and the rest of the periodic table – into the ether. New generations of stars formed from the flotsam and jetsam of these stellar shipwrecks.
The Milky Way is a spiral galaxy, a stirred broth of stars, a gruel thick and slab circling round about within the charmed cauldron of space, and it is ancient – nearly as old as the universe itself. Our solar system formed within an outer arm of the Milky Way approximately 4.6 billion years ago. It began as a nearly flat, swirling accretion disc of gas, rock, and ice. As the dense center of the disc collapsed into a ball of plasma and ignited into our Sun, the rest condensed into planets and moons and planetesimals. Small terrestrial planets took shape in the interior, giants of gas and ice further out.
The third planet, Earth, found itself at an agreeable distance from the Sun. It was warm enough to allow for oceans of liquid water. It was also massive enough to keep a grip upon an atmosphere, and its relatively large moon helped stabilize its orbit.
Organic molecules, the building blocks of life, formed from inorganic components in chance chemical reactions, and rained down during lightning storms in the pre-biotic atmosphere or bubbled up from the early oceans following meteor strikes. Biomolecules appeared like morning dew across the face of the planet.
And one day in the scalding hot water along a deep sea vent, or on a sandy beach contaminated by radioactive minerals, or just under the surface of a sea warmed by the Sun, a chain of organic molecules combined into a new molecule that could replicate itself. Perhaps nucleic acid arose from a random combining of amino acids, slipped into a vesicle of fatty acid, and the first proto-cell was born. Or perhaps organic matter was taught how to duplicate itself by inorganic matter – upon clay crystals or within foam bubbles along sea shores. We don’t know the exact nature of the abiogenesis process that spawned life on Earth. It might have started in rocks deep below the planet’s surface, or deep in outer space. But once on the scene, units of self-replicating matter, in little steps, through small feats of chemistry, grew in complexity until chemistry became biology, and, from humble beginnings, life arose.
The first replicated molecules were rough copies; they contained many variations. Most of these new molecules likely lacked their parents’ ability to replicate themselves, but some inherited the talent, and a few of the new molecules even replicated themselves better or possessed other miniscule enhancements that allowed them to produce more offspring. Improved forms tended to spread and therefore to have the opportunity to improve further. This process of natural selection continued in successive generations, and the new forms became increasingly elaborate and intricate. Evolution was underway.
Fossils of colonies of photosynthetic bacteria found in sedimentary rock suggest that by 3.4 billion years ago microbial life was already prolific. The longest reign by far of any life form on earth was not that of the mammals or of the dinosaurs, but of teeny-tiny, single-celled organisms. The Pre-Cambrian period, during which the only living things were microscopic, lasted over 3 billion years (about six times longer than all the periods that followed, combined).
Gluttonous blue-green algae feasted on sunlight and burped up oxygen, which was absorbed by the oceans until the oceans could take no more, and then oxygen built up in the atmosphere. The careless fools changed the composition of the air they breathed and they choked to death on their own waste. Their folly set the stage for new, oxygen-loving forms of life to emerge.
The eukaryotes, single-celled organisms containing nuclei and organelles, evolved over the last third of the Pre-Cambrian. Consider that single-celled organisms have much shorter life cycles than do human beings. There are bacteria whose generations last but 15 minutes. It is said that one year to a human is the equivalent of seven to a dog. How long must a billion years be to a primitive eukaryote! Why did it take so long for these cells to evolve from more primitive cells?
Because eukaryotes are astoundingly complex. They’re too small to see with the naked eye, and yet each one is like a majestic city state. They’re surrounded by city walls (the plasma membrane) within which a busy citizenry of even smaller components operate and together form a thriving economy. Small as they are, they’re comprised of trillions of molecules and are ten times wider than the prokaryotic cells from which they sprang. The mitochondria within them may have evolved from aerobic bacteria that were swallowed, but not consumed, by the cells. Mitochondria contain their own DNA and reproduce like bacteria by dividing in two. Eukaryotes may have started as two organisms living in an endosymbiotic relationship, the smaller making a home inside the body of the larger.
Eukaryotes were the crowning achievement of the Pre-Cambrian. When they arrived on the scene, and especially when they learned to cooperate with each other, anything became possible. Evolution exploded.
Colonies of eukaryotic cells discovered the advantage of sticking together. Subsets of cells specialized into organs, allowing multi-celled organisms to attach to the sea floor, or wiggle along it, or undulate in the waves above. New beasties emerged sporting innovative new features: shells, skeletons, teeth, tentacles, eyeballs, fins, brains. Plants and fungi took root on land, and soon fishy, four legged pilgrims crawled from the water to claim this new territory. Insects also took to the land and then to the sky.
The large reptiles had their day in the sun, until the sun was blotted out by a dust cloud from a catastrophic meteor impact, the eruption of a super volcano, or a combination of such events.
As the dust settled, little, furry, bright-eyed varmints scurried from their holes to survey the transformed landscape. They had evolved from therapsids, mammal-like reptiles, and had lived alongside their dinosaur cousins for eons. They were warm-blooded and relatively big brained, which allowed them to adapt quickly to changing conditions when other animals could not. Now they were free to roam wherever they wished and to fill the ecological niches that had been vacated by the reptiles. Soon enough the oceans were overflowing with whales and seals; the land was laden with rodents, hoofed herbivores, and the carnivores that hunted them both; bats crisscrossed the sky; and, in the trees, curious primates watched the unfolding scene.
And so we come to humans.
In his 1980 television program “Cosmos: A Personal Voyage,” the astrophysicist Carl Sagan told a science-based version of the Genesis story, infused with the sense of wonder for which he became famous. (The story was the inspiration and the model for this chapter.) Of the rise of humans he said, “Star stuff, the ash of stellar alchemy, had emerged into consciousness.”
The most abundant elements in the universe are hydrogen, helium, oxygen, neon, nitrogen, and carbon. The main elements in the human body are oxygen, carbon, hydrogen, and nitrogen. Notice the overlap. When we look to the stars, we look in a mirror; in a very real sense what we see is ourselves. We bear an atomic kinship with the rest of the universe.
Our cells are comprised of organic molecules that in turn are comprised of atoms. These atoms were created within ancient stars. “Star stuff.”
But let us trace our pedigree a little further. Before they were trapped within stars and fused into heavier elements, the atoms in our bodies were smaller particles, and before that, pure energy, joined together with the entire cosmos in a single point many times smaller than the head of a pin.
The energy was released. Energy begot stars, stars begot heavier elements, heavier elements begot the Sun and the Earth and living beings, and these beings evolved the intelligence to look up at the universe, and in some little way to understand it.
The best holy books are full of poetry. The King James Bible, the Bhagavad Gita, the Koran, the Tao Te Ching, the Dhammapada. We are by nature poetic beings. Sagan wrapped up his Genesis story with a conclusion about humankind founded upon reason and evidence, but also beautifully poetic:
We are a way for the cosmos to know itself.
- Amino acids, which bond together to form the proteins and nucleic acids in living cells, have been discovered in abundance within meteorites (e.g. the Murchison meteorite), and spotted by radio telescopes in deep space (see http://www.newscientist.com/article/dn2558-amino-acid-found-in-deep-space.html). They may have been generated when meteors struck the ammonia-rich oceans (see http://www.wired.com/wiredscience/2008/12/in-a-vat-primor). They can be generated in a simple laboratory experiment from ordinary gases merely by zapping them with electricity. (Although it is questioned whether the gases used in the classical Miller–Urey experiment would’ve been common in the pre-biotic atmosphere, a study published in 2011 revealed that one of Miller’s electric discharge experiments on gases that would’ve been common in volcanic plumes produced a surprising array of amino acids (see http://www.pnas.org/content/early/2011/03/14/1019191108.full.pdf+html).)↩
- The Wikipedia article on Abiogenesis was extremely helpful to me in writing this paragraph, and is probably the first place to go to learn more about the current scientific theories on how life on Earth began: http://en.wikipedia.org/wiki/Abiogenesis#Gold.27s_.22deep-hot_biosphere.22_model.↩
- See here: http://www.ncbi.nlm.nih.gov/pubmed/20528193 and here: http://www.ucmp.berkeley.edu/bacteria/cyanofr.html.↩
- Admittedly, it’s oversimplifying to say that eukaryotes were slow to evolve only because of their relative complexity. See the following article for reasons why evolution was slow in the Pre-Cambrian: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC44277/?page=3.↩