For decades, we have wondered whether Mars, Venus and the icy moons of Jupiter and Saturn might harbor evidence of past or present microbial life. Yet how did life start here? Because earth is a living biosphere with weathering and very active geology, there are no easy answers.
Over billions of years, our planet’s crustal plates have collided and crushed up against each other like SUVs in a crowded Black Friday parking lot. As a result, geological evidence of ancient life here is hard to come by.
The oldest almost universally accepted evidence of life is the 3.48-billion-year-old West Australian stromatolites (layered sedimentary fossils usually formed from cyanobacteria), Keyron Hickman-Lewis, a microbial paleontologist at Birkbeck College at the University of London, told me in his U.K. office.
But biology on earth may have even started several hundred million years sooner.
In 2015, UCLA geologist Elizabeth Bell and colleagues reported carbon isotope ratios measured in a carbon inclusion within a 4.1-billion-year-old zircon crystal from the Jack Hills in Western Australia, says Hickman-Lewis.
It lends credence to the idea that there was biological processing even before the formation of the earliest cells.
Four billion plus years ago is an incredible number. Given that our own solar system is some 4.6 billion-years old, life on earth may have formed within a few hundred million years after our planet’s formation.
As for where life here first emerged?
I generally favor the seafloor over terrestrial settings, although whether life emerged in localized seafloor hydrothermal vents or within a regionally or globally important hydrothermal–sedimentary environment at the seafloor remains subject to debate, says Hickman-Lewis. Hydrothermal activity was certainly more significant on the early earth so the localities in which life might have emerged were likely far more common than at present, he says.
Why study young microfossils?
Young fossils give us a ’time zero’ for interpreting more ancient examples, which are often highly matured and very altered, says Hickman-Lewis. The microfossils we study in specific recent environments —- such as Argentina and Ethiopia —- have many similarities to what we anticipate ancient stromatolites would have looked like at the time of formation, he says. We can compare the morphologies and geochemistries of young fossils with ancient examples to understand how the ancient examples may have been preserved over billions of years, says Hickman-Lewis.
Isolated Lakes
In Argentina, Hickman-Lewis and colleagues studied samples from a lagoon in the Cari Lauquen Lake in Mendoza province. And in East Africa, he studied samples from around Lake Ashenge, a highland lake in southern Ethiopia.
Both lakes are quite young, not more than a few hundred thousand to a million years (hence the excellent preservation of microfossils), says Hickman-Lewis.
How difficult is it to find direct evidence for microfossils here on Earth?
Many of the oldest traces of life have extremely simple morphologies that can be easily reproduced by other natural processes, says Hickman-Lewis. So, we really must find microfossils that have been preserved over billions of years that are extremely well contextualized environmentally and geologically, he says.
Geographical Locations
The locations in which we currently find ancient stromatolites are quite likely not the geographical locations in which they were deposited, says Hickman-Lewis.
Even so, there has been progress in using paleomagnetism to track the geographic origin of these rock samples.
Ancient Rocks
These rocks were magnetized by an ancient magnetic field, Planetary scientist Benjamin Weiss, director of the Paleomagnetism Lab at MIT, told me via email. The dip of this magnetization relative to the ground can tell us the latitude at which they formed, says Weiss. This because the dip of Earth’s magnetic field increases with latitude; it’s horizontal at the equator but vertical at the poles, he says.
We published a paper with such a measurement on the stromatolites in the Strelley Pool Chert in Australia’s Pilbara region, says Weiss. Our measurements show that these stromatolites formed within 8 degrees latitude of the equator, he says.
What’s most puzzling about life’s origins here?
Geochemical analyses of the oldest rocks have given us an increasingly clear picture of the geological characteristics of the earliest earth, says Hickman-Lewis. But we’ve yet to bridge the gap between the geological and chemical contexts required to explain the crucial steps between prebiology and biology itself, he says.
Learning From Mars
In truth, we may have to look to Mars for context on the origin of life on earth. That seems totally counterintuitive. But unlike earth, the Martian surface is much older simply because it lacks the sort of weathering and active tectonics that can wreak havoc on a planet’s geological record.
What are we missing?
In terms of the origin of life, what we’re missing is direct evidence of the environmental and geological contexts in which this happened, says Hickman-Lewis. I hope that this information can possibly be filled following a Mars sample return, he says.
The Bottom Line?
We know that on average Mars’ surface is much older than earth’s, says Hickman-Lewis. So, he says it’s conceivable that there are horizons on Mars that date from the time of life’s origin on earth. I hope in the future we can use some of the information we get from Mars as a tool for understanding earth in deep time, says Hickman-Lewis.
