In 1952, when Stanley Miller and Harold Urey recreated the primordial soup in their lab and zapped inorganic elements into amino acids, neither could have foreseen the next few decades’ progress towards determining life’s origins. At the E.E. Just Science Symposium this past weekend, Dartmouth Professor of Physics and Astronomy Marcelo Gleiser, the last person to interview Miller before his death, spoke about this progress in his lecture titled “The Origin of Life.” He described the “Frankenstein revisited” experiment, which led to the birth of astrobiology, a young science with the lofty goal of explaining how life arose in a largely unlivable universe. Astrobiologists divide the history of the universe into four ages: the physical age (spanning the Big Bang to the formation of the first stars), and the more complex chemical, biological, and cognitive ages.
After the formation of the first stars, planetary systems were created in the chemical age. Dying stars flooded space with more complex elements and radiation, fusing matter into planets and forming galaxies with the help of gravity. As Gleiser says, “The first stars were the true alchemists.” With the help of gravity, this stardust formed the first planets, among them Earth.
A few hundred million years of chaos followed Earth’s formation 4.5 billion years ago. Once the debris from an asteroid collision shaped itself into a moon, however, Earth’s tilt stabilized, giving the planet seasons, liquid water, and a chance to enter the biological age. Comets delivered water to the surface, and oceans swelled alongside magma to tides made colossal by the nearby moon. A crust solidified as the moon drifted away, and circulation in the Earth’s core created the ultraviolet-blocking magnetic field. Around this time, star-born inorganic matter, including hydrogen, nitrogen, and carbon, formed simple molecules such as methane and ammonia, although whether this happened on Earth or in outer space is unknown.
The chaos subsided around 3.5 billion years ago, allowing for the synthesis of organic molecules into amino acids and sugars. Then, in a step that has yet to be fully grasped, life was born. Hypotheses about life’s beginnings abound. Some say metabolism came first; coacervates (fat droplets) were believed to sustain chemical reactions in their protective bubbles, shielding them from the early atmosphere. Others, including most biochemists and geneticists, say life began with RNA. The “RNA world hypothesis” aims to explain the origin of DNA by taking a step back, with only one catch—RNA is nearly as complex a concept.
At any rate, the life forms that sprang up 2.5 billion years ago owe their survival to prokaryotic cyanobacteria, known to most as blue-green algae. Cyanobacteria photosynthesized the atmosphere’s abundant carbon dioxide into oxygen, creating the ozone layer that shields the Earth’s surface from the sun’s ultraviolet rays. The tempering of the atmosphere led to the formation of eukaryotes—complex life. Armed with mitochondria, the source of which is another mystery, these organisms became multicellular after millions of years of mutation.
If we were watching the process all over again, at what moment would we recognize life? If life means reproduction, then are viruses, fire, or stars alive? Not according to the most widely accepted definition of life: a “self-sustaining chemical network capable of Darwinian evolution.” A virus may perform life-like functions, but once it detaches from its host cell, it becomes, once again, a crystal. We think of life as evolving on a ladder, with nowhere to go but up. In reality, evolution has no agenda; life, in our case, resulted from accidents, such as the asteroid that wiped out the dinosaurs and let mammals emerge from their burrows. As Gleiser says, “Natural selection had no plan to create an elephant out of a bacterium.”
Intelligent life, the hallmark of the cognitive age, may have arisen in more than one place. The laws of physical science dictate that the chance of finding life on one of the billions of stars in one of the billions of galaxies is astronomically high. Biology, however, has stricter standards. If we find any extraterrestrials, they are likely to be simple-celled and underground. In an age when many eyes turn to the universe for answers, however, Gleiser stresses Earthling uniqueness. “We are the minds of the cosmos,” he says. “We likely are not the most advanced beings in the universe—for all we know, humans liven up some alien civilizations’ space “Petri dish”—but, for all we know, we are the universe’s “moral center.”
