At 10:58 in the morning of September 28, 1969 a fireball appeared in the sky above Murchison, Australia, a farming town 167 kilometers north of Melbourne. Residents reported hearing a tremor and said that afterwards the town was suffused with an organic odor that reminded them of methylated spirits. In the days that followed, people searched the countryside to find fallen pieces of the meteorite. More than 100 kilograms of material were recovered, over four times the amount the Apollo 11 astronauts had brought back to earth from the first lunar landing nine weeks earlier.
Like those lunar rocks, the fragments of the Murchison meteorite contain traces of extraterrestrial matter far older than what earthbound geology — with its ever-recycling crust of tectonic plates — can offer. A 2020 chemical analysis of a sample from the meteorite found that its oldest particles may be seven billion years old — two billion years older than our solar system.
The Murchison fragments have offered key insights about the kinds of chemicals that were abundant in the younger universe, making it a rich resource for scientists who study the origins of life on earth. A 2010 study of Murchison samples identified 14,000 molecular compounds, including 70 amino acids, and suggested that the full meteorite might contain millions of distinct organic compounds. When the scientists mixed dust from the sample with water as part of the analysis, they reported that the resulting paste smelled like rotten peanut butter.
The moon, for its part, smelled like gunpowder, according to the Apollo astronauts, and organic compounds have proven quite rare in lunar samples. The moon itself was created, according to the current leading hypothesis, as the result of a collision between the proto-Earth and a Mars-sized body — an ultimate “sterilizing event” around 4.5 billion years ago that provides the far bookend of the period in which life as we know it could have emerged on earth. (The other bookend is provided by the oldest commonly agreed-upon fossil microorganisms, which are around 3.5 billion years old).
When and where, during this billion-year window, were the conditions right for life to first emerge? Charles Darwin imagined, in an 1871 letter to Joseph Hooker, some ancient “warm little pond” with just the right combination of ammonia, light, heat and electricity present to generate self-replicating proto-life capable of evolutionary descent. Since then, proposed locations for Darwin’s pond have since included hot springs like those at Yellowstone National Park, and undersea thermal vents. In 1952 chemist Stanley Miller moved the pond to the lab bench, showing that complex organic compounds could be assembled from inorganic predecessors using an approximation of conditions on primitive earth — a groundbreaking proof of concept that advanced the field of early life studies (although its assumptions about early earth conditions have since fallen out of favor).
THE BOTTOM-UP APPROACH
H. James Cleaves, a geochemist at the Tokyo Institute of Technology’s Earth-Life Science Institute, outlines the current state of early-life studies in The Origins of Life, a research review commissioned by the John Templeton Foundation. Available at full length and in summary form, Cleaves’ overview looks at the main approaches to early life studies. The “top-down approach” looks backwards, using molecular phylogenies to discern traces of ancestral life from present-day lifeforms. The “bottom-up” approach focuses on what processes occurred in primitive environments to contribute to a complexification of chemistry eventually leading to systems that could undergo some kind of heritable evolutionary process.
Today every living thing is composed of four basic ingredients — proteins, fats, sugars, and the nucleic acids DNA or RNA. Proteins (chains of amino acids) carry out work within the cell, fats form the cell membrane, sugars provide fuel, and DNA or RNA supply the instructions needed to develop, survive and replicate. The Murchison meteorite shows us that several of those ingredients may have already been available by the time that life emerged. The meteorite’s many identified compounds include sugar derivatives and nucleic acid bases, and potentially small amounts of fatty acids. One study showed that cell-boundary-like structures formed spontaneously from organic extracts of the meteorite.
“For all the uncertainties surrounding the emergence of life, it appears that the formation of a ‘prebiotic soup’ is one of the most firmly established aspects of the primitive Earth,” Cleaves writes, “though its recipe remains difficult to decipher.”
Read the full white paper, The Origins of Life: A Review of Scientific Inquiry, written by H. James Cleaves at the Tokyo Institute of Technology’s Earth-Life Science Institute.
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