The colonization of land by life pushed back in time


Fossile lifeA recent story on Science Daily made me stop and read with fascination. It’s about the discovery of fossils that showed life colonized land more than two billion years ago. That’s a shocker, because all indicators are that the Earth was a hostile place, land was barren, and life was a lot simpler.

But apparently we underestimated the ability and tenacity of life. This gives us hope that life may exist on other planets or moons.

Mars, at least its surface, is unlikely to harbour life now, but may have in its water-rich past. We may find fossils. Europa, Jupiter’s ice-covered moon, may have life in the oceans under its surface. Finding out will be tough. Titan? A remote possibility. But these fossils give us new hope: life can survive in very hostile conditions.*

Conventional scientific wisdom has it that plants and other creatures have only lived on land for about 500 million years, and that landscapes of the early Earth were as barren as Mars. A new study, led by geologist Gregory J. Retallack of the University of Oregon, now has presented evidence for life on land that is four times as old — at 2.2 billion years ago and almost half way back to the inception of the planet.hat landscapes of the early Earth were as barren as Mars.

That raises several interesting ideas. First, what sort of life was it? The researchers don’t know for sure, but it’s a bit like slime mold or some lichen. It was tiny, but rather complex given its age and a promising candidate for the oldest known eukaryote. Sci-news notes:

Diskagma buttonii are very small – about 0.3 – 1.8 mm long. They most resemble modern soil organisms called Geosiphon, a fungus with a central cavity filled with symbiotic cyanobacteria.

It was something not quite animal or plant – something simple. But not that simple: they appear to have had symbiotic cyanobacteria in their interiors. Cyanobacteria appeared about 3 billion years ago, and started photosynthesizing (with water as a reducing agent) and making oxygen between then and about 2.6 billion years ago (life itself first showed up about 3.5 billion years ago but some estimates push it much earlier, to 4.3 billion).**

That presence suggests symbiosis. Symbiotisis is common today, even your own gut houses symbiotic bacteria, and the mitochondria in most eukaryotic cells – including those in our own bodies – are probably endosymbionts that became absorbed into the cells, ending their independent existence, somewhere long ago in the evolutionary process. These new finds suggest symbiosis was already evolved 2.2 billion years ago!***

On the University of Oregon site, it describes them:

The images enabled a three-dimensional restoration of the fossils’ form: odd little hollow urn-shaped structures with a terminal cup and basal attachment tube. “At last we have an idea of what life on land looked like in the Precambrian,” Retallack said. “Perhaps with this search image in mind, we can find more and different kinds of fossils in ancient soils.”

For a more technical description, see here.

This find offers an answer to some of the questions around the “Great Oxygenation Event,” roughly 2.4 billion years ago. At that time, most life was very primitive, but there was a lot of cyanobacteria, which used photosynthesis for food production. As any plant owner knows, one of the positive side effects of photosynthesis is the production of oxygen as a waste product. Great for us oxygen-breathing creatures.

But back then, most life was anerobic and oxygen was a toxic gas. Oxygen was less than 5% of the atmosphere then, and most was absorbed by metals and minerals (oxidation). All those cyanobacteria were producing enough oxygen to wipe out most of their competitors. A big change in the nature of life occurred as the oxygen content grew and creatures struggled to evolve to adapt to this poison or die. A huge wave of biodiversification blossomed.

Just as a comparison, today the level of oxygen is at 21%. That increase may have been in part thanks to these tiny creatures and their (as yet undiscovered) co-creatures colonizing the barren land at such an early stage in our planetary history.

Retallack said

“This gains added significance because fossil soils hosting the fossils have long been taken as evidence for a marked rise in the amount of oxygen in the atmosphere at about 2.4 billion to 2.2 billion years ago, widely called the Great Oxidation Event.”

These odd, primitive creatures lived on land earlier than it was previously believed the land was colonized. It would be almost 2 billion years until a vertebrate walked the land, though – in the Middle Devonian, 395 million years ago (MYA), although arthropods scuttled over the land about 530 MYA (they were probably terranauts which retreated to the oceans after their forays). What’s hard to grasp is the enormous time between these creatures and more complex-multi-cellular animals (starting around 580 MYA).

Seeing those tracks frozen in the rock, one cannot help but think of T.S. Eliot’s poem, The Love Song of J. Alfred Prufrock:

I should have been a pair of ragged claws
Scuttling across the floors of silent seas.

Diskagma buttonii, however, never scuttled. It attached itself to a surface and probably never moved. It would take two billion years – until the Ordovician, about 480-460 MYA – before recognizable plants would start to colonize the land, spreading over the ages from shorelines to interiors.

Exciting stuff, this “evidence of paleoproterozoic terrestrial life” – perhaps one of the most significant finds in paleontology in decades. Ain’t science grand?

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* Who says scientists have no sense of humour? The creatures have been named “Diskagma buttonii,” meaning “disc-shaped fragments of Andy Button.”  Button is a character from a British TV series, Episodes, played by Joseph May. I’ve never seen it, but now I am curious.

** Traditionally, it was assumed life began in the oceans, somewhere around 3.5 billion years ago, but perhaps much earlier. However, new research has suggested it may have began on land – in mineral-rich geothermal pools on land, then migrated to the water. Darwin’s “warm little pools.” But there’s also a theory that microbial life existed on land almost as early as it did in the water. In part this is difficult to say for sure because the life was soft-bodied and did not leave many fossils, and much of the sedimentary rock of that age has been subsumed into the mantle.

*** Plants have developed an interesting evolutionary path, and have relationships with all other life that are not necessarily symbiotic but are important nonetheless, including with fungus, a mutually beneficial relationship that is at least 400 million years old. See here also.