Life evolved on Earth surprisingly quickly after our planet’s birth – almost as soon as it was possible. But for something like three billion years, that life had a boring physical sameness: tiny cells called bacteria and archaea were the only game in town. If you had landed on Earth during those hundreds and hundreds of millions of years, you would not have seen much other than a big blue ocean sporting some rather rocky, unpromising, and unimproved real estate.
Those three billion years must have really felt like three billion years.
At some point, the long creative dry spell came to a halt: eukaryotes evolved. These life forms (of which you are one) contained genetic and structural innovations that permitted them to grow much larger and more complex than bacteria or archaea, and were likely the first individual organisms large enough to see with the naked eye. Eyes had not actually evolved yet, but you take my meaning.
When, exactly, this happened has been the subject of great debate. A newly discovered cache of fossils resembling modern red algae may push back the earliest known such life by a staggering 400 million years to a whopping 1.6 billion years ago (in the press release announcing the findings, the grad student who found the fossils claims she was so excited upon discovering them she had to walk around her building three times before she told her supervisor). The findings were published in the journal PLOS Biology back in March by a team of Swedish scientists based on fossils discovered in central India.
Whether or not their hypotheses concerning these organisms' identity ultimately prove to be accurate, it is fun to look at them and ponder the long-dead living beings that created them over a billion and a half years ago.
The fossils are embedded in stromatolites – finely layered petrified mounds of light-harvesting bacteria. Under the microscope, the bacteria appear as “masses of entangled filaments”. But scattered among the usual filaments were two kinds of gigantic filaments.
The first, named Rafatazmia chitrakootensis by the scientists, measured a whopping 58 to 175 micrometers across. A typical bacterial filament measures more like seven.
Some of the Rafatazmia cells contain intriguing furniture: a mystery rhombus suspended by a tether. To the authors of the paper, it recalls a similarly-shaped structure called a pyrenoid found inside many algae today. In red algae, pyrenoids put the sugar produced by photosynthesis into storage in the form of long chains we call starch. The most early-evolved red algae alive today have a single, centrally located polygonal pyrenoid whose angular shell is constructed from plates of starch.
Other Rafatazmia cells seem to contain pores in the walls between cells while others have lumps in the same spot. Modern red algae also have pores in the walls dividing newborn cells. This hole is later filled by a characteristic, lump-like “pit plug”. No modern bacteria or archaea contain such structures.
It is possible that the rhombuses and putative pit plugs are simply junk created during fossilization and don’t actually represent structures that existed in these cells when they were alive. However, younger (though still quite old) fossils demonstrate that cell nuclei and even chromosomes can be accurately fossilized. The rhombuses also appear consistently in the center of the cell, in a consistent shape, with a consistent orientation, strongly suggesting they are real organelles, the authors posit.
A second fossil called Denaricion mendax appears to be nothing more than a featureless tube from the outside. But a special x-ray microscopy that allows us to peer inside reveals a stunning interior: structures resembling stacks of poker chips or coins.
There is little additional structure. Although the overall size of the filament – 130 to 275 micrometers across – is highly suggestive of a eukaryote, some living bacterial cells do reach gargantuan proportions, so size alone cannot be definitive. It is also important to look at cell volume, and here Denaricion may fall short.
A logarithmic chart comparing cell width to cell volume shows that Rafatazmia clusters with modern eukaryotic green algae called Spirogyra, while Denaricion overlaps with cyanobacteria called Beggiatoa and Oscillatoria.
Oscillatoria, in particular, bears a strong resemblance to Denaricion.
Bacteria are size-limited because diffusion – a passive process resembling a random walk -- is the only way they can move stuff around the cytoplasm of their cells. Supersized bacteria are huge not because their cytoplasmic volume is large, but because they contain enormous chemical storage tanks with a thin veneer of cytoplasm plastered between the tank and the cell membrane.
Eukaryotes, possessing complex internal skeletons built of microtubules, actin filaments, and intermediate filaments that can act as package-delivery superhighways, face no such constraints, a major reason they can get so much bigger.
As Denaricion’s cells get wider, they also get thinner – hence the poker chip look. This tendency of Denaricion to maintain a relatively small cell volume suggests they may be bacteria or archaea in spite of their fat filaments.
The third fossil type was particularly intriguing, appearing not as filaments, but as a cluster of lobes. The cells inside are arranged in what appears to be a “tissue”, or a group of cells that adjoin each other on many sides and usually function with a common purpose. Tissues were the first architectural step on the path that led to the gothic cathedrals we call plants and animals today.
Dubbed Ramathallus lobatus, this organism was enormous for its time and is easily visible to a human. Individual lobes exceed 3 millimeters and collections of lobes are more than a centimeter wide. They radiate from a point where the organism likely anchored to the stromatolite. In cross section, the organism looks a bit like a slice through a brain.
Sometimes the cells were arranged in what the scientists described as “fountains”. Such structures often occur when filaments grow in bundles that fuse to create a tissue.
Scattered among the tissue were clusters of four cells the scientists dubbed “tetraspores”. Such shapes are reminiscent of the products of sexual cell division, meiosis, in modern red algae. Only eukaryotes perform this DNA shuffling maneuver that promotes genetic novelty, but whether these cell clusters were actually tetraspores or even the result of meiosis is not clear.
Ramathallus is reminiscent of an organism called Thallophyca that occurred a billion years later, the authors say. In that billion years of time, not much advancement in morphological complexity appears to have occurred, at least based on the fossils we’ve found so far. But the apparent billion-year architectural stagnation was not to last.
Around 600 million years ago, large life blossomed into a suite of experimental forms called the Ediacaran biota. Within 100 million years, that world too was largely or wholly replaced by the large forms of the Cambrian Explosion, many of whose descendants populate Earth today.
Why did it take so long to go from the first lobes to the first leaves or livers? Why was life on Earth so easily evolved but so slow to create structural complexity? These fossils contain no obvious answers. But if these life forms really are the ancestors of modern red algae, it means that many of the seaweeds that litter beaches around the world like so much sea trash may deserve a lot more eukaryotic respect. They are Eldest, and they are still here, in spite of everything.
Bengtson, Stefan, Therese Sallstedt, Veneta Belivanova, and Martin Whitehouse. "Three-dimensional preservation of cellular and subcellular structures suggests 1.6 billion-year-old crown-group red algae." PLoS biology 15, no. 3 (2017): e2000735.