“Early Earth may not have been as hot and hellish as we thought…” suggests a recent, popular article in the New Scientist after opening with the question of whether life may have begun “in the freezer.”
Whether it’s a discussion of climate change, possible benefits of high or low temperature on the body, frozen compared with fresh food, or the reason for upper respiratory infections in winter, science stories dealing with effects of temperature can always be used to get people debating. The media love portraying issues in terms of this, that, or its opposite, even high quality media from the New York Times to the New Scientist, know that conflict, or the perception of conflict, makes for an interesting story.
Things are good or bad, hot or cold, and so when a study came out suggesting that Earth’s Archaeon Eon was not so hot, but actually cold, it was an opportunity to write headlines implying that everything we’ve been thinking, everything that researchers have been publishing, is now wrong. But a closer look at even the summery articles containing the hyped headlines would show that the real story was different.
In this case, the topic was not Earth’s climate in recent history, but way, way back, early in geologic time, when the fossil record shows evidence of early life. In addition to water and organic matter, another requirement for abiogenesis — the origin of a living system from pre-biotic chemical evolution — is an energy source. The earliest organisms had to make their own fuel, as plants and photosynthetic microorganisms do today. A plausible way of doing this is to utilize high energy inorganic chemicals that are ejected through volcanic vents on the ocean floor, known as the hydrothermal vents.
Deep sea organisms do this today. In fact, entire ecosystems exist deep in the ocean near the hydrothermal vents, where microorganisms—and possibly some exotic multi-cellular organisms– make their own food just as trees make their own food, except instead of sunlight they use high energy chemicals that come out through the vents. Called chemoautotrophs, such organisms become food for other deep sea organisms that don’t have the ability to make their own food (heterotrophs), but some kind of autotrophs are needed at the bottom of the food chain, and for the origin of life.
Utilizing chemicals from the hydrothermal vents
One leading hypothesis in origins of life research is that Earth’s earliest organisms were chemoautotrophs. One of the best places to be a chemoautotroph is around the hydrothermal vents, so one hypothesis is that this is where abiogenesis could have occurred on Earth. If you really oversimplify this, the implied take-home message is that a hot environment was both the location for the chemical evolution leading to to the first life and the habitat for the earliest living cells. Thus, when Maarten de Wit and colleagues at Nelson Mandela Metropolitan University in South Africa published a study recently suggesting that ocean temperatures during the time of the origin of Earth life were low—in the range of 20-40 degrees Celsius—the summary in popular media ended being that Earth was not so hot and hellish; it was a freezer.
That’s exaggeration when it comes to the cold as well as the heat. With respect to the latter, there is an early time on Earth that geologists equate with Hell. It’s called the Hadean eon (named for Hades, Greek god of the underworld) and it runs from the formation of the Earth just under 4.6 billion years ago until roughly 4 billion years ago. Certainly, this was a hellish time, but the Hadean eon is not when we think that life got started on Earth. Rather, it took place during the next eon, known as the Archean, beginning when the planet’s crust had cooled down significantly so that there could be oceans. During Archean eon, the complex, organic molecules needed to form membranes of cells, enzymes for running metabolic processes, and enormous polymers for storing and using genetic information would not fall apart. Within just a couple hundred million years or so after crust cooling—really just a smidgen in geologic time—there’s evidence in the form of chemical isotopes in rocks that life may have existed on Earth. A few hundred million years later there are fossils of single-celled life forms, so almost as soon as the Earth cooled from its Hadean infancy, that’s when life emerged on Earth (or possibly took root from organisms delivered here by meteoroids). Before appreciating the new research from South Africa, we must be clear that we never thought that Earth was “hot and hellish” during the Archean eon, when life began. To say that in an article or headline is to conflate the Archean and Hadean eons.
Archean eon: Balmy, but not hellish
Based on proportions of different chemical isotopes in rock samples from 3.5 billion years ago, most studies published over the last couple of decades have suggested that the nascent ocean of the Archean eon was, something like 85 degrees Celsius on average. That was much warmer than the seas we know today, but very cold compared with the earlier Hadean eon when oceans and life were not possible.
The benefit of the hydrothermal sea vents is the supply of high energy chemical compounds, but that does not mean that the transition to life, or the environment of early life forms, had to be in warm water. Today, the average ocean temperature is a little below 20 degrees Celsius (usually 17 degrees is cited). That’s cold, but at the hydrothermal vents water temperature approaches 400 degrees C. It doesn’t boil, because of the high pressure at the bottom of the ocean, but it contrasts sharply with the more livable temperature of ocean water that’s not near the vents.
A little colder than we thought but not a freezer
As mentioned earlier, the finding by the South African team was that ocean temperature 3.5 billion years ago was in the range of 20-40 degrees Celsius. For perspective, normal human body temperature is 37 degrees C, and when that’s the temperature outside its bikini weather. It’s much lower than the 85 degrees that we’ve been thinking, but it’s higher than ocean temperature of the current time.
As noted in the New Scientist article, the Archean ocean being 20-40 degrees instead of 85 implies that Earth may have been engulfed in glacial ice from time to time way back then. However, it doesn’t mean that the Earth was a freezer and doesn’t mean that abiogenesis took place in the ice.
Rather, it suggest that there was a contrast of temperature environments. There were very high temperatures deep down at the hydrothermal vents and colder temperatures overall and what the new study does is to tweak the value of the colder, non-vent water. Whether the average ocean temperature was as high as 85 degrees or as low as 20, it was much colder than vent water, so there were were gradients of temperature. Either way, there could have been early organisms utilizing the inorganic chemicals to generate energy, but not living in those super hot temperatures.
Significance for astrobiology
The implication from the new study that the origin of life could have taken place when the parts of the ocean were cold enough to have ice may not merit freezer and Hell headlines that it has received, but it has marvelous implications for astrobiology, the search for life on other worlds. Considering worlds in the outer Solar System that are surrounded by an ice layer with an underlying ocean — Jupiter’s moon Europa, Saturn’s moon Enceladus, and maybe the dwarf planet Ceres — the new finding from South Africa is particularly intriguing. And maybe that’s enough to forgive the extra enthusiasm regarding the popular descriptions regarding the temperature of Earth’s Archean ocean.
David Warmflash is an astrobiologist, physician and science writer. Follow @CosmicEvolution to read what he is saying on Twitter.