Last week, we wrote about the fundamental three questions concerning the origin of life on Earth: When? Where? How? Although they are interrelated, each has a specific set of sub-questions that keep researchers very busy.
Astrobiology, the study of life's origins on Earth and the possibility of life elsewhere in the cosmos, is a burgeoning scientific field that has the characteristic of being essentially interdisciplinary. It needs a concerted effort from astronomers, geologists, biochemists, physicists, geneticists, paleontologists, planetary scientists, biologists, etc., all contributing their share to unveiling one of science's greatest mysteries: How did life begin on Earth and could it exist elsewhere?
On the question of "when," we saw that it may have many possible answers, and that we may never be sure we know the correct one. Life could have started and then become extinct several times before it took hold on early Earth. The best that we can do is to collect convincing evidence of its first steps, left as biomarkers in very old rocks. This effort has narrowed the window for life's baby steps to being at least 3.5 billion years ago (bya). Given that Earth is about 4.5 billion years old and the Late Heavy Bombardment ended around 3.9 bya, this leaves a window of about 400 million years for life to have emerged and taken hold here. Not a long time — at least in geological terms.
The question of "where" depends, of course, on the environment of early Earth. By environment we mean the composition of the atmosphere, the acidity and salinity of the primal oceans, the existence of shallow or even dry expanses of land with fairly calm conditions. After formation, Earth had no atmosphere. Slowly, gases from volcanos got trapped and a mix of hydrogen sulfide, methane and about 10 to 200 times more carbon dioxide than today made up an atmosphere very different from the one we have now. After about 500 million years, as the Earth cooled, water vapor was brought into the mix. Oxygen was there only as part of compounds. The sun was only about 70 percent as bright as today; greenhouse gases kept Earth from freezing.
From 4 bya to about 2.5 bya, there was hardly any free oxygen in the atmosphere. But, then it happened. Somewhere, somehow (the where and how questions) life emerged as chemicals self-organized to become an autonomous network of reactions capable of metabolism and reproduction. There are many competing ideas as to where life emerged first. In a letter to his friend Joseph Hooker from Feb. 1871, Charles Darwin speculated that life may have started in a "warm little pond":
"It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present.— But if (& oh what a big if) we could conceive in some warm little pond with all sorts of ammonia & phosphoric salts —light, heat, electricity &c present, that a protein compound was chemically formed, ready to undergo still more complex changes, at the present day such matter would be instantly devoured, or absorbed, which would not have been the case before living creatures were formed."
A primordial chemical soup leading to first life! For this to happen the right ingredients must mix in the right place. Life needs proteins, long chains of amino acids. This is what Darwin means by his mixing to form proteins from ammonia, phosphoric salts, heat and electricity.
In 1953, Stanley Miller, then a graduate student at the University of Chicago, persuaded his adviser Harold Urey to perform an experiment simulating Earth's early atmosphere and composition. Starting with simple inorganic compounds (methane, ammonia, water and hydrogen), Miller used sparks to simulate electric activity — lightning — known to have been present abundantly during volcanic eruptions. The results were amazing: More than 20 amino acids were produced, proving that the steps from simple inorganic to complex organic compound were feasible.
With the discovery of living creatures in deep ocean hydrothermal vents, a competing theory suggests life could have happened in one or more of these exotic sites, where no oxygen is needed by primate organisms to survive. Some claim life needs a high concentration of chemicals to get jump-started and that watery environs are too diffusive: Things spread out before they have a chance to find one another to react. For this reason, some scientists suggest clays as a possible cradle for first life, while others consider the possibility that tidal pools could also do the job given that concentrations rise and fall rhythmically. Another possibility is to combine several of these environments, in landlocked thermal vents.
There may be more than one answer, of course. Given the pervasiveness of life, surviving and even thriving on environments as varied as deep ice, hydrothermal vents and radioactive pools, it seems reasonable to consider that life as an experiment may have many versions, taking advantage of different conditions and different chemistries. Remarkably, once life emerged here on Earth, it worked to change the environment in radical ways. The early creatures, mostly in the form of cyanobacteria, mutated into photosynthetic bacteria, releasing large amounts of oxygen into the young atmosphere. As a result, the atmosphere became oxygen-rich, allowing for more complex metabolic reactions to take place. About 530 million years ago, during the so-called Cambrian Explosion, life exploded into incredible diversity: the biological big bang!
We owe our presence on this planet to these hard-working photosynthetic bacteria — a sobering thought, considering our self-importance as the dominant species here. Life not only depends on the environment where it emerges but also works to change this environment. What we have learned from life's early steps shows that it may flourish under very adverse and diverse circumstances. These discoveries make the possibility of life on other planets or moons much more tangible, even if odds in our solar system are still low.
Given the huge number of worlds in our galaxy alone, trillions of them, circling hundreds of billions of stars, it would be a true statistical fluke if Earth were the only harbor for life. That leaves us with the question of life's complexity — Are we the exception or the rule? — one that we will take up next week.
A previous version of this piece incorrectly stated that "after about 500 billion years, as the Earth cooled, water vapor was brought into the mix," when it should have said "500 million years." It has been corrected in this version.