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."