HATCHERIES: A group of scientists proposes the pools formed form vented geothermal steam, pictured here, may have provided the perfect environment for life's early cells. Image: Courtesy of National Park Service
Earth started as a violent place, its surface churned by continuous volcanic eruptions and cloaked in an atmosphere that would have been poisonous to today's life-forms. Furthermore, the thin primeval atmosphere may have provided only scant protection from the young sun's harsh ultraviolet glare. Given these inhospitable conditions, scientists have long wondered: How did the first cells come to be nearly four billion years ago?
Conventional scientific wisdom holds that life arose in the sea. But a new study suggests that the first cells—or at least the ones that left descendants still extant—got their start in geothermal pools, like those seen at Yellowstone National Park and other geologic hot spots today. The argument rests on one indisputable observation—enzymes common to all archaea and bacteria are built from potassium, phosphorus or zinc, not sodium.
Some biologists suspect that the membranes of early life-forms were not yet the tight coverings that they are today, and would have instead let small molecules and ions flow in and out freely. If life arose in the salty sea, then the first cells and their living relatives might be expected to have enzymes built from abundant sodium—or at least tolerate more sodium internally. That modern archaea and bacteria instead possess internal fluid low in sodium, and enzymes built from other elements hints that they arose in an environment both rich in such elements as well as relatively sodium-free. "If the very first membranes were leaky for small molecules and ions, then the interior of the first cells should have been in equilibrium with their surroundings," explains biophysicist Armen Mulkidjanian of the University of Osnabrück in Germany, lead author of the paper presenting the hypothesis published online February 13 in Proceedings of the National Academy of Sciences. "By reconstructing the inorganic chemistry of the cytoplasm, it might be possible to reconstruct the habitats where the first cells could dwell."
The team noted that most modern cells maintain a high ratio of potassium ions to sodium ions. "We looked all over the place for the conditions and processes that would lead to [potassium] enrichment," Mulkidjanian says. The only such places extant today are so-called "vapor-dominated" geothermal systems—locales where water, heated deep within Earth until it becomes steam, reaches the surface, cools and condenses back to elementally enriched liquid pools. Condensed geothermal steam in these pools can have ratios of potassium to sodium ions as high as 75 to 1, and are rich in the other elements of life that have been leached from rock by the hot water. Thus, Mulkidjanian and his colleagues argue that they may have been the "hatcheries" of the first cells.
The argument matches a perhaps prescient suggestion from Charles Darwin in an 1871 letter: "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, etcetera present that a protein compound was chemically formed, ready to undergo still more complex changes." Nobel laureate and geneticist Jack Szostak of Harvard University has also argued that the first cells probably had leaky membranes and that early oceans were not a favorable environment for the origin of life.