21 Sep—Life
How life began remains a mystery. The Earth began as a lifeless rock but ended teeming with life. How did life rise from non-life?1
Lepidus: Your Serpent of Egypt, is bred now of your mud
by the operation of your Sun: so is your Crocodile.
Anthony: They are so.
— Anthony and Cleopatra, Act 2, Scene 7
Ironically, until the rise of modern science, we wouldn't have asked this question. The everyday creation of life from non-life was commonly accepted in Christendom. People believed, for example, that mice came from soiled rags, maggots from putrid meat, and snakes and crocodiles from river mud.
So entrenched was this belief that when experiments began to show that all life comes from other life, other scientists and laymen alike defended the spontaneous generation of life as an obvious fact. It wasn't until Louis Pasteur's famous experiment in 1859 that the idea of ongoing spontaneous generation was laid to rest.
Omnia omnino animalia—etiam vivipara, atque hominem adeo ipsum—ex ovo progigni. (Absolutely every animal—including those who give birth to live young, including man himself—are born from an egg.) — William Harvey
But that gave rise to the question: if all life comes from other life, where did the first living thing come from? Life must have come from non-life at least once.
Many hypotheses have been put forward to answer this question (too many to cover in detail), but none have gained a consensus. The mystery remains. We are unsure whether life began in shallow pools of water, radioactive beaches, undersea volcanic vents, below the Earth's surface, or even in outer space. Very basic organic chemicals could have been created in any of these locations, and thereafter life could have arisen from these building blocks.
Most hypotheses rely on an atmosphere devoid of free oxygen. Free oxygen reacts strongly with organic molecules and would have destroyed these materials on the early Earth. Early life relied on an environment that would be deadly to us today.
One prominent hypothesis proposes that life began when RNA formed from those building blocks. Most of us are familiar with DNA, the molecule that stores our genetic information, but RNA may be less familiar. RNA is a very closely related molecule to DNA.
Unlike DNA with its famous double helix, RNA is usually single stranded. RNA can also replicate itself whereas DNA needs assistance from other molecules. Because DNA can't copy itself, it is unlikely that life began with DNA. It is thought that RNA could have preceded it as the beginning of life.
Unfortunately for RNA's career as our future genetic code, it is less stable than DNA. It couldn't link together in long chains that contained enough information to code for complex life.
Once molecules of RNA formed and began to multiply through replicating themselves, the fundamental requirements for life were met. At some point, RNA could have handed over the job of storing genetic information to DNA, but RNA stuck around to help DNA reproduce.
But that is only one hypothesis among many, and it doesn't begin to address the question of how the genetic material got into cells.
However it happened, life had probably begun by about 3,800 million years ago—September 21st on the Cosmic Calendar—and made its way into cells by 3,500 million years ago. The earliest fossilized life is found in clumps of bacteria called stromatolites.
Life had begun.
(Note that human evolution took much longer than 2 million years, contrary to what Alan Watts suggests in his lecture.)
1. The question of how life began is separate from the question of evolution of species. Organic evolution requires that life already exists and says nothing about how life began.