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.
Leaky proto-cells in the sea, Mulkidjanian and his colleagues note, would have much higher exposure to sodium than potassium, even near hydrothermal vents on the ocean floor, making it difficult to maintain any imbalance between the two. But that does not mean that the cells necessarily arose in a potassium-rich environment. In fact, geothermal areas in the modern world are usually highly acidic and thus deadly. "It could still be that cells evolved the ability to generate and maintain a high [potassium-to-sodium] ratio in their cytoplasm for functional reasons," Szostak notes. "The basic question is whether the observed high K–Na ratio reflects the historical environment in which life originated or underwent early evolution, or instead reflects some underlying chemical necessity, such as better functioning of certain cellular components."
Furthermore, life started on an Earth that may not have had continent-size landmasses but rather a series of archipelagos formed by volcanoes, much like the islands of Japan today. As a result, the water cycling through these areas may have been very different, notes marine chemist Jeffrey Bada of the Scripps Institution of Oceanography at the University of California, San Diego.
And then there is the fact of evolution itself. Cellular life has changed the makeup of its internal fluid—the cytosol—countless times over the eons, and modern-day life-forms exhibit a wide variety of compositions. "Is it not at least equally likely that they have modified their cytosolic compositions to accommodate their cytosolic functioning once they had control over this process, which all modern cells do?" asks geochemist Jim Cleaves of the Carnegie Institution of Washington. "Any modern environment which matches this composition would then be purely coincidental." In fact, Cleaves argues it may be impossible to tell what early life—or even the first universal common ancestor of life—was like, given all the intervening evolution. It's akin to trying to "infer an abacus from a modern PC," he notes. "You might be able to infer a TRS-80, but then it all gets a bit hazy and there might be no vestigial remains of the intervening stages of biological evolution."
But life has preserved some things down through more than three billion years of evolution: for example, the shielding of enzymes and other internal cellular workings from oxygen to allow them to operate. Of course, the early Earth's atmosphere lacked oxygen, instead it was rich in other gases, such as hydrogen sulfide. "This is the same smell that you can find on a trip to Yellowstone National Park, where [hydrogen sulfide] seeps from the mud pots, geysers and other underground exit sites," Mulkidjanian notes. The first cells evolved in such a place and "their progeny carried the affinity for such an environment from mother cell to daughter cell through the past three billion years."