How did life on Earth begin? Answering this question has been a top priority in exobiology and astrobiology for more than 50 years.
And how close are we to an answer?
Here’s the good news: over the last 50 years or so, scientific understanding of what life (on Earth) is and is not, what life requires, what limits environments impose on life has grown by orders of magnitude.
Another side of this happy story is that, as a result of our vastly improved understanding of life on Earth as well as the creation and evolution of Earth and other planets and their place in the cosmic environment, scientists are now considering multiple viable theories about how life might have originated on this planet.
These theories are reviewed in an excellent essay published in the January 17 issue of Science magazine. Biochemists Jimmy Gollihar and Andrew Ellington of the Center for Systems and Synthetic Biology at the University of Texas and Matthew Levy of the Department of Biochemistry at Albert Einstein College of Medicine begin their review by observing, “The origin of life remains a daunting mystery in part because rather than knowing too little, we increasingly know about too many possible mechanisms that might have led to” life on Earth.
“It is possible that it is not a knowledge of prebiotic synthesis that is wanting, but knowledge of prebiotic replication.” Thus, a number of research groups are focusing on synthesizing a cell that can replicate itself – no easy task, to put it mildly.
Origin of life theories have waxed and waned in prominence over the years. The results of Stanley Miller’s experiments of the 1950s, simulating presumed atmospheric conditions on the early Earth – supported the theory of abiogenesis – the origin of life on Earth from non-living matter. Space research over the next few decades showed that prebiotic compounds are present in the cosmos and that the complex organic components of life could have been delivered to Earth by comet and asteroid impacts. Deep-sea hydrothermal vents are under study as analogues to early-Earth environments that may have facilitated the origin of life. There’s the theory that mineral surfaces may have served as templates for the organic chemistry leading to life. Another theory is that life – single-celled life – may have originated elsewhere – say, on Mars – and been delivered to Earth by meteorites. And so on.
NASA astrobiologists recently published the results of a new round of testing of meteorite samples for prebiotic organic compounds. They detected amino acids – the building blocks of proteins – in a 360-microgram sample of a meteorite (that’s 360 millionths of a gram).
Meanwhile, “Exploring martian habitability” is the subject of a special section of the January 24 issue of Science, reporting on a year’s worth of findings of the Mars Science Laboratory (MSL) mission. In an introduction to this section, John Grotzinger, head of the MSL science team, says “early Mars was habitable, but this does not mean that Mars was inhabited.” What the MSL team is now aiming for is a better understanding “of how organic compounds are preserved in rocks.” A better understanding of these processes could help the MSL team “to narrow down where and how to find materials that could preserve fossils as well.”
There’s a lot of work to do on Mars. And we still don’t have a definitive answer to the question of how life began on Earth. However, we surely do have a lot of interesting research questions to answer.