I hope I never see one.
But I find it charming anyhow
That someone thought we were one.
Let me be upfront with you: this is the story of a beautiful theory, slain by an ugly fact. The theory lived fast, died young, and left a gorgeous corpse.
A purple earth was born from the reflections of Shil DasSarma. Why, he wondered, does chlorophyll-based photosynthesis, the source of most fuel on earth, not take advantage of the main peak of sunlight? Surely it would offer an advantage to use that. He then noted halobacteria have purple membranes that do absorb in that band. Moreover the halobacterial photosynthetic system looks primitive — it has many fewer components and shares a mechanism with other energy systems of pumping protons out of the cell to create a battery. Here we go: These purple synthesizers were first, and the johnny-come-lately cyanobacteria had to make do with what was left of the solar spectrum.
It sounds plausible at first: the simpler organisms started things off, and the complicated ones developed something more powerful and outcompeted the primitive ones. On top of that — Purple Earth Theory! A name so eye catching that a music group took it.
It was so eye catching that Live Science picked up the idea in the abstract and made a decent popularization of the hypothesis. It sums up the proposal itself and adds in the relevant background to help people understand the significance of it (if true). Yet oddly, that was where the purple earth theory ended. Even the link in the Live Science article to American Scientist, a good source of popularized science, is dead. Why so little interest in such a radical notion?
I might have been more suspicious if I had recognized the name of David Des Marais. His mild critique was
While that looks on the surface like a counter speculation, Des Marais, unlike DasSarma, has decades of research into cyanobacteria and microbial communities. The mildness is deceiving.
The closer one looks, the less there is there in the purple earth hypothesis. While it is true that halobacteria can, under carefully controlled conditions, do photosynthesis anaerobically, they use arginine to do it. Arginine, an amino acid. A complicated amino acid. Who among us believes the early earth was a soup of arginine?
But wait! It gets worse. Yes, halobacterial photosynthesis is much simpler than cyanobacterial photosynthesis. It uses retinal (yes, like the molecule in your eye) instead of the baroque chlorophyll. But retinal synthesis requires oxygen.
Our oxygen comes from chlorophyll-based photosynthesis, when organisms split water to steal electrons and spit out the nasty oxygen. There is no conceivable way that a global ecosystem of halobacteria preceded the cyanobacteria. None.
The purple earth hypothesis was dead on arrival. It’s just that apparently no one noticed (except possibly Des Marais). Where’s the gorgeous corpse, then? Dead as the hypothesis is, it nonetheless leaves behind two key concepts.
First, we need to keep an open mind about exoplanets and their life. Some of them might easily be purple. A red dwarf, which most stars are, puts out very little light and most of it in the red frequencies. An efficient photosynthesizer there, scarfing up all the red light, might look purple under our flashlights.
Second, our own earth might itself have been purple for a while — from cyanobacteria. Recent extensive genetic studies suggest that the cyanobacteria that we know, that have left bacterial mats and stromatolites for three and a half billion years, spring from procyanobacteria which were simpler than ours and did not have oxygenic photosynthesis at first. If they upped the levels of appropriate pigments to cope with the high levels of UV light washing down before the ozone layer formed, the earth might have been…purple.
THE abstract
The other photosynthetic bacteria that use chlorophyll likely gained it by lateral gene transfer from cyanobacteria.
Overview of cyanobacterial evolution
Detailed genomic analysis of cyanobacteria
Another view of procaryotic LGT for photosynthesis
Purple bacteria could have made earth purple. But genetic studies suggest they got photosynthesis from cyanobacteria by lateral gene transfer.
Timeline of photosynthesis on earth
DasSarma is still talking to the media.
Aquarium folks know cyanobacteria can be purple.
Lots on bacterial mats
A beautiful video by Des Marais on contemporary bacterial mats as models for early earth or Mars:
The rest of DasSarma's scientific work