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When Cells Discovered Architecture

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In early 1997, while still a freshman in college pondering whether to study biology or archaeology, I opened up my copy of Discover Magazine to find an article that startled and captivated me. "When Life Was Odd", read the headline, and if that didn't sell me, the photos did. They were of Ediacarans, creatures named after the stony hills in southeast Australia where they were first recognized in 1946, and their puffy, weirdo bodies were wonderful.  

There was the bilaterally radiating ovoid disk of Dickinsonia, the sea pen-like fronds of Rangea and Charniodiscus, the fruit loop embedded in a pineapple ring of Cyclomedusa, and the wickedly curved and utterly gorgeous vortex of Tribrachidium. Much later, I discovered the glorious shark tooth kebabs of Fractofusus from Mistaken Point, in Newfoundland, Canada. Imagine: a whole world of sea life that looked like almost nothing alive today that once briefly ruled the oceans – only to vanish forever. What an exciting thing!  

But what were these things? No one was really sure. They seemed somehow inflated, or pneumatic – a state that persists in few, if any, creatures alive today. They immediately pre-dated the Cambrian explosion – the sudden flowering of animals that produced the lineages that led to jellyfish, cockroaches, T. rexes, and giant sloths. Clearly, that was an important event, but what about this fascinating overture? Did us big bad higher animals wipe out the poor, defenseless, balloon animals? Or did we, at least in part, evolve from them? And where did they in turn come from -- other than out of the blue?  

Scientists are still in the process of answering all these questions. But that last one got much more interesting after a paper in Nature published in February – to relatively little fanfare -- debuted an entirely new group of Ediacarans, a group that predates classic Ediacarans by a good 20-40 million years. A group with a few members that look – surprisingly – like Stuff Alive Today. A group, it turns out, that might have evolved just after Earth emerged from the controversial cataclysm called "Snowball Earth" – and could perhaps even have been prodded to evolve by it. A group that, as far as we know, are the earliest surviving fossils of algae, plants, and possibly animals on Earth. They are called Lantians. And they are wonderful too.

Previously, the oldest known Ediacarans were about 580 million years old –  about 40 million years older than the Cambrian. In attempting to explain their pillowy appearance, scientists have speculated they may have had photosynthetic algae in their tissues, or have fed by absorbing nutrients across their thin bodies. They seem to have lived in tidal mudflats or shallow tidal pools and were preserved when they were covered by sediment, though some, like the Avalon group, were abyssal creatures eking out a living by osmotically absorbing dissolved nutrients.

Until the discovery of the Lantians, the only recognized pre-Ediacaran massively multicellular fossils were stromatolites, blue-green algal high-rises of limited morphological sophistication: they are simply the left-behind houses of cyanobacterial biofilms stacked layer upon layer. They've been around for at least 3 billion years. And after they evolved . . . not much happened. Life barely changed, at least in terms of shape and size, for a really long time. Until something really big did happen: Earth froze solid. Or slushy. Or possibly with a band of water around the equator, or in some other odd configuration. Or maybe not at all. However it may have been, based on a near-worldwide appearance of glacial deposits, some geologists think our planet iced up.  

Those who say that it did believe the planet froze three times for millions of years each in the late Proterozoic, in the aptly named Cryogenian period. This age was followed by the last period of the Proterozoic: the Ediacaran, which stretched from 635 to 542 million years ago. The Cambrian, in turn, followed that. Until now, the oldest known Ediacaran fossil group was called the Avalon, and first appeared about 575-580 million years ago.  

But in eastern China, north of Nanjing, lies a layer of rock called the Lantian formation made of a flaky mudstone called shale. Inside are fossils – the newest, and earliest Ediacarans, scientists now report. Though many were discovered, described, and named over 10 years ago, they were misdated to 551 million years old. In the Nature paper, scientists at the Chinese Academy of Sciences and Northwest University in China and at Virginia Polytechnic Institute correct this, estimating their age at 607 to 635 million years.  

The shale lacks any cross-banding or wave or current induced structure. These rocks formed in quiet water.  Most of the fossils appear to have been attached to the seabed. And the fossils found inside are randomly oriented and not folded or broken. They weren't swept there by currents or deposited from above. They seem to have died, quietly, where they lived. The authors suggest that was a muddy bottom below the base of storm waves but still within reach of the sun.  

Together, they form a buffet of biodiversity. Unlike other Ediacarans, some of these fossils stun by their sameness, not their eccentricity. One dichotomously branching form(see photo at top) looks very much like modern algae (scroll down to photos here). One fossil (see photo at left) resembles resembles the modern sea palm, Postelsia palmaeformis..

 

 

 

 

But others remain enigmatic. One may be a polyp-like creature. The axial traces of others [right] might be the digestive tract of worm-like organisms with "probosci", or alternatively, another alga with "holdfast" and "stalk". This strange chain could be anything [below, left].

All told, the scientists estimate the Lantian fauna consists of about 15 types of mostly complex form. They could be 15 different species, or some could be different developmental stages of others. Strikingly, there seems to be no species overlap – perhaps even at the kingdom level, according to Guy Narbonne in a commentary accompanying the Nature paper -- with the abyssal Avalon biota, the previous record-holder for oldest big life. Avalon is filled with "traditional" pneumatic Ediacarans. Lantian is populated by large algae. That has both to do with the differences in habitat, but also the details of the different preservations (Avalon is sandstone).  

And here's the truly intriguing bit. The layer containing the fossils lies directly on top of (conformably, for the geologists in the crowd) the rock layer with the deposits from what we think was the last glaciation of Snowball Earth. Thus it may well be that oxygenation of seawater and other changes in ocean chemistry spurred by the massive global glaciation pulled the trigger on the development of "big life". 

Of course, the Lantian holds the current record for oldest Ediacaran fossils, but that does not mean they were, and they almost certainly were not. As with all of the fossil record, some life was preserved by chance but most wasn't. And of that that was preserved, much may have eroded out and thus lost over the last 600 million years, while much may still lie exposed unseen or buried deeply across and within the planet.  

And there remains one discrepancy. Early organisms – photosynthetic or not -- are thought to have had a strict “no oxygen, no respiration, no life”  policy. But the rocks these fossils lie in say the Lantian oceans were devoid of oxygen. To account for this, the authors believe that the oceans were in the process of oxygenating, and that for short periods of time – too short to have been properly sampled and detected so far -- the normally stifling Lantian seabeds lay under oxygen-rich water. These new life forms, not knowing any better, snuck in to take advantage of prime uninhabited real estate. But the oxygen soon vanished, suffocating, preserving and consigning them to muddy graves. Graves from which we have finally retrieved them, 600 million years later. If they truly were algae, they would be glad to finally again see the light of day.  

Sources:

An early Ediacaran assemblage of macroscopic and morphologically differentiated eukaryotes .

Evolutionary biology: When life got big .

"Prehistoric Life"

 

About the author: Jennifer Frazer is a AAAS Science Journalism Award-winning science writer who lives in Boulder, Colorado. She has degrees in biology, plant pathology/mycology, and science writing, and has spent many happy hours studying life in situ. Read her blog The Artful Amoeba and find her on Twitter at @JenniferFrazer.

The views expressed are those of the author and are not necessarily those of Scientific American.

The views expressed are those of the author and are not necessarily those of Scientific American.

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