Life may have crawled out of the sea 700 million years earlier than anyone thought--although crawled is not quite the right word.
In the beginning the land was barren, and the ocean cradled all life. From about 3.5 billion years ago, when simple, single-celled organisms first appeared off some primeval shore, until 3 billion years later, when the first plants greened the land, life was strictly a maritime affair. That, at least, is the conventional view of life’s early history. But if Paul Knauth, a geochemist at Arizona State University, and Robert Horodyski, a paleontologist at Tulane, are right, it is a view that may be in for a sea change. They believe they’ve found fossil evidence that life, albeit extremely primitive life, thrived on land as far back as 1.2 billion years ago, nearly 700 million years earlier than anyone had believed.
Knauth first picked up the trail of these possible ancient landlubbers almost ten years ago, high in the mountains of central Arizona. He and a graduate student, Mark Beeunas, were chipping away at 1.2-billion- year-old rocks, looking not for fossils but for somewhat more indirect evidence of early life. Plant life, Knauth knew, always leaves behind a telltale sign of its presence, even in the absence of tangible fossils. When photosynthesizing plants or microbes use sunlight to power the chemical reactions that transform carbon dioxide and water into simple sugars, they selectively absorb from the air carbon dioxide composed of carbon 12 rather than the heavier isotope carbon 13. When plants die and decay, or even while they respire when living, they enrich the soil with carbon 12.
In Arizona’s Sierra Ancha, Knauth and Beeunas found rock strata-- limestone overlain by quartzite--marked with the carbon-12 signature typical of that left by plants or photosynthetic microbes. Here, they believed, was evidence for early life on land. Oddly enough, the limestone was originally a marine deposit. But the carbon-12-enriched upper layers of the limestone appeared weathered and had what looked like sinkholes in them; the unweathered rock below contained no excess carbon 12. Knauth and Beeunas decided that a collision of two tectonic plates must have lifted the marine limestone above sea level, exposing it to air and to weathering sometime before the carbon-12 residue was formed by terrestrial organisms 1.2 billion years ago.
To confirm this idea, the researchers turned to another set of isotopes: those of oxygen, and more specifically, the oxygen bound up in water molecules. When the sun evaporates water from the ocean surface, vapor made of lighter oxygen isotopes rises into the air more readily; heavier isotopes get left behind. Rainfall made of this water eventually saturates rocks on land with the lighter isotopes. The weathered rocks that Knauth had cut out of his limestone formation bore the light-oxygen stamp, indicating that they had once been exposed to air.
That meant the carbon-12 enrichment had happened to the rocks on land. Knauth and Beeunas searched for fossils to prove their belief that terrestrial life was much more ancient than anyone had previously suspected, but they came up empty-handed. Even so, they thought they’d found good circumstantial evidence. We published a paper back in ’84, says Knauth. We said there must have been something growing there. But it was kind of ignored. Everybody wanted to see the fossils.
Now Knauth finally has something to show the skeptics. In January he and Robert Horodyski announced that they had found the long-sought fossils. Over the years Knauth and another graduate student, Ray Kenny, had continued to comb the Arizona site as well as another formation near Death Valley in California. Last year, while examining rock samples from both locales in his lab, Knauth thought some spherical and tubular shapes in a few of the slabs looked suspiciously like fossil microbes.
He sent some samples to Horodyski, an expert on microfossils. Horodyski cut them into paper-thin sections and examined them under a microscope. After much searching through many samples, he confirmed Knauth’s suspicions. I found one section that had extremely well preserved microfossils, says Horodyski. They were tubular filaments. They were intertwined as if they were part of a mat.
The fossils resemble cyanobacteria, primitive, single-celled organisms that, unlike plants, lack a cell nucleus--but that are capable of photosynthesis. Some cyanobacteria form filaments and mats, and indeed some much more ancient marine fossils, the strange, layered rocks called stromatolites, have been thought to be mats made by cyanobacteria or their relatives. Knauth and Horodyski believe that microbial mats probably covered large sections of the land surface more than a billion years ago because Knauth has found a hefty carbon-12 enrichment over large areas of the Arizona and California formations. I think the carbon isotopes are telling us that there were a lot of them, says Knauth. We’re not talking about a little thin scum or green stains on the rocks.
Not all researchers in the field are convinced by Knauth and Horodyski’s data. Some think the microbes may really have been water dwellers--that they may have lived in lakes rather than on exposed rocks. The discovery that life spread from the ocean to freshwater lakes 1.2 billion years ago would still be news, but not nearly as surprising as the discovery that it established itself on dry land.
But if Knauth and Horodyski are right, and life did have a firm foothold on land more than a billion years ago, it could undermine the conventional wisdom that the biggest changes in evolution, including the switch from single-celled to multicellular organisms, happened in the ocean. The oldest known fossils are stromatolites from Western Australia that are probably marine in origin and that date back 3.5 billion years. For 2 billion years after that, however, life remained very simple and single-celled. Knauth thinks it may be that salt water was actually hostile to the development of more complex forms of life. Why for all this length of time was there nothing more complicated on Earth? he asks. It seems to me that the sea is a terrible place to evolve life, particularly in the evaporitic environments--such as the shallow bays that are often taken as evolutionary cradles--where you get a concentrated brine that will pickle everything.
We have this idea that life originated and evolved in the sea, and then about 500 million years ago land plants got established, and then things crawled up out of the sea and started eating them. That idea may be in for some revision.
