Rob Sheldon: How we know the 558 mya animal Dickinsonia remains really contained fats

Recently, some readers asked whether the recent Dickinsonia fossil “fats” find from 558 mya featured cholesterol. Our physics color commentator Rob Sheldon explains further:

Cholesterol was not found by these researchers, nor did they make announcements of soft tissue in a fossil. What they did find were the breakdown products of cholesterol called “sterols”. Plants make phytols that break down similarly. There might be hundreds to thousands of breakdown products of these biochemicals. When these materials are run through a mass spectrometer, the device sorts them by chemical weight. Really good mass specs (like the ones I used to design for NASA) can even separate isotopes of carbon and hydrogen. Then a simple molecule like CH4 might have four or five peaks for molecules with different weight isotopes of C-14, C-13, and C-12, combined with H or D. When plotted with mass on the X-axis, the profile looks like a skyline of a city, with some combinations much more abundant than other combinations. Each complicated biomolecule has a particular skyline shape that is stored in a computer. When a measurement is made, thousands of potential skylines are added together to reproduce the observation, and sometimes there can be ambiguity in the reconstruction. Higher resolution mass specs can usually remove the ambiguities. It was a high resolution mass spectrometer just like this that caught Lance Armstrong’s artificial testosterone doping, because testosterone from plants has more C-13 than testosterone from people.

I want to emphasize just how sensitive these mass spectrometers are. When looking at C-14, they detect parts per trillion–in Carbon alone. That is, they detect 1,000,000,000,000 C-12 atoms for every C-14 atom that is present. When looking at molecules, the sensitivity drops down into the parts per billion or parts per million. This is still very impressive, and orders of magnitude more sensitive than chemical assays. As an aside, what limits the C-14 chronometer to 65,000 years is not the 5500-year half-life of C-14, but the fact that other rare processes are making miniscule amounts of C-14 continually, which swamps the original signal of ancient atmospheric C-14.

So what the researchers did in this report was to scrape some rock from the layer with the Ediacarian critter and run it through the mass spec. They also scraped some rock from above and below the fossil, though I’m not positive how they got their samples; it could have been a solvent extraction, a powder extraction or even a laser that zapped the rock. The point is that they were able to get a profile of mass spec analyzed sterols of different rock layers. Above the fossil layer they detected plant sterols, which abruptly transitioned to animal sterols in the fossil layer. This was indicative of both the animal and its habitat/food source. It also acts as a control since most forms of contamination (including bacterial sterols) would not show such a sharp transition. Furthermore, the sharp transition gives information about heat and metamorphic history of the rock.

I repeat, there is no need to find pristine cholesterol in the rock. It merely had to have the “fingerprint” of animal sterols as distinguished from plant sterols.

The rest of us are watching the animated Dickinsonia graphic below. And asking, would they have named it “Dickinsonia” if they had known it would be a star someday?:

Rob Sheldon is the author of Genesis: The Long Ascent

See also: Fats recovered from Ediacaran fossil, 558 mya, shows that animals then were “large,” “abundant” Rob Sheldon: Both straight-chain fats (crude oil), and cyclic hydrocarbons (coal) survive for millennia in the ground. It isn’t too surprising if cholesterol (4 rings, 5-carbon chain) can survive. The principle way oils degrade is through oxidation, and buried in the mud is a good way to avoid oxygen.