Evolution of Evolution — Text-only | Flash Special Report
Origin and Evolution of Life on a Frozen Earth
Scientists debate whether life’s start was hot or cold
By John C. Priscu
The origin of life on Earth is one of the most debated issues in science. Despite ideas put forth by early philosophers, it was Charles Darwin who first posed an explanation for life's origin that complemented his evolutionary theory of life on Earth. In a letter written in 1871 to botanist Joseph Hooker, Darwin envisioned:
“It is often said that all the conditions for the first production of a living organism are present, which could ever have been present. But if (and Oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., 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.”
Darwin’s “warm little pond” idea was supported experimentally by two University of Chicago researchers in the early 1950s, Stanley Miller and Harold Urey, who showed amino acids, the building blocks for protein, could be formed when electric shocks were introduced to a flask of water containing the gases methane, hydrogen and ammonia. Although efforts to understand the origin of life have been hampered by lack of direct evidence, these early experiments led many to believe that life on Earth had a “hot start.” The discovery of hot-loving microorganisms, called thermophiles, supported this concept. Subsequent study of thermophiles led to the conclusion that the last common ancestor evolved in a hot environment and prompted speculation that life originated not in a warm pond, but in a very hot one. This hypothesis is still ‘hotly’ debated by researchers who believe that such classifications are strongly biased by the use of a single gene for the construction of the tree of life. They argue that deep branching tells us nothing about what came before the common ancestor, and that hot temperatures are not compatible with the stability of structural and functional components of living cells that existed before the advent of life.
Recent discoveries of cold-loving microbes, dubbed psychrophiles, living in solid ice have extended the known boundaries for life on Earth and provided the basis for new theories on the origin and evolution of organisms on our planet. Data obtained over the last 10 years have shown that bacteria inhabit polar ice sheets as well as temperate glaciers. Little information exists on atmospheric and geological conditions during the period when life originated on our planet, some three to four billion years ago. However, we do know that the luminosity of the sun was about 30 percent lower during this period, producing what could have been a subzero world. The half-lives of prebiotic molecules are much longer near the freezing point of water compared to half-lives at the boiling point. Stability in these precursor molecules is essential to the development of the molecular complexity required to initiate life.
Darwin’s original ideas of a warm little pond are based to some degree on a habitat that can produce a high concentration of prebiotic molecules. Freezing concentrates molecules, allowing for a high probability of self organization into more complex molecules, while at the same time, reducing the potential to degrade the molecules. The mineral surfaces within ice veins, and inclusions associated with impurities also provide a scaffolding to assist with the synthesis and assembly of complex molecules. Recently, experiments have shown that simple monomeric molecules concentrated in ice veins for almost 30 years can produce precursors for nucleic acid bases.
Though more research is required to determine whether life originated in hot or cold environments—or both independently, it is highly probable that cold environments have acted as a refuge for life during major glaciations. Around 600 million years ago during Earth’s Neoproterozoic Era, early microbes endured an ice age with such intensity that even the tropics froze over. According to this “Snowball Earth Hypothesis,” the Earth would have been completely ice-covered for 10 million years or more, with ice thickness exceeding one kilometer. Only the hardiest of microbes would have survived this extreme environmental circumstance, and perhaps icy refuges may have served as oases for life during these lengthy crises. The concentration of microbes within ice veins in this frozen environment would favor intense chemical and biological interactions between species, which would entice the development of symbiotic associations, and perhaps influence the development of more complex life-forms through evolutionary time. As such, these ice-bound habitats provided opportunities for microbial evolution, and the acquired biological innovations may have triggered the Cambrian explosion, or the seemingly sudden appearance of most major groups of complex organisms, which occurred immediately after this snowball Earth event.
Clearly, there is growing evidence in favor of a cold origin of life on our planet and future biological research on icy environments will provide more clues to the origin and evolution of life on Earth, as well as other icy worlds. Although the lack of evidence from ancient Earth means we may never know precisely how life began, Darwin’s warm little pond hypothesis certainly played a seminal role in molding current notions on the subject. Darwin appeared cognizant of this fact when he added the following line in his 1871 letter to Joseph Hooker: “It is mere rubbish thinking at present of the origin of life; one might as well think of the origin of matter.”
John C. Priscu is a professor of ecology at Montana State University in Bozeman, Mont. He is a leading expert on polar ecology with 25 years of work in Antarctica where he has studied life associated with the ice sheets as well as sea and lake ice. He investigates ecological processes in these environments that allow organisms to survive and evolve over long temporal and broad spatial scales. The National Science Foundation has supported his research since its inception.
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