What will be our final trace?
I met a traveller from an antique land
Who said: Two vast and trunkless legs of stone
Stand in the desert. Near them, on the sand,
Half sunk, a shattered visage lies, whose frown,
And wrinkled lip, and sneer of cold command,
Tell that its sculptor well those passions read
Which yet survive, stamped on these lifeless things,
The hand that mocked them and the heart that fed:
And on the pedestal these words appear:
“My name is Ozymandias, king of kings:
Look on my works, ye Mighty, and despair!”
Nothing beside remains. Round the decay
Of that colossal wreck, boundless and bare
The lone and level sands stretch far away.
The legacy of Ozymandias provides an interesting metaphor for the fossil record. It is easy to recognise a living entity while it is still alive. It can be a little difficult to say that something was alive if we only ever saw it dead, and it becomes even harder to claim that something was alive if there is very little of it left. Time will eventually reclaim everything. So, what are the minimum traces that life leaves behind?
Biochemicals.
One group of biochemicals of particular interest are carotenoids. As the name suggests, carotenoids can be found in carrots – the carrot colour is ‘β-carotene’. Carotenoids are found elsewhere as well, and critically, they are only made by plants, algae, bacteria, fungi and one animal, an aphid. Carotenoids are not made by geological processes, but are instead evidence for life. Like all traces of life, carotenoids are eventually broken down and recycled by Earth processes. A plant rich with carotenoids will die and settle into the Earth, and even without the aid of bacteria, the atoms in the carotenoid will disassociate and break the biochemical apart. So, eventually these minimal traces of life will eventually fade away. These biochemicals break down into smaller and less complex molecules. So, carotenes in the fossil record, or carotanes – the breakdown products – are unambiguous signals that life once existed, even if those remnant molecules are all that remain. Like Ozymandias’ “…vast and trunkless legs of stone…”
Pigments like β-carotene are evidence for life, but these biochemicals can be altered during diagenesis.
So, when we investigate the time worn sediments from an aeons-old Earth, or Mars, we can look for carotenoid degradation products as vestiges of life. This was the focus of a 2010 study by Craig Marshall and Alison Olcott Marshall. Diagenetic alteration can result in “…hydrogenation of the polyene chain…”, the long carbon backbone of the carotenoid that gives the molecule it’s colour. In essence, the unaltered carotenoid has many carbon atoms bound to each other with ‘double’ bonds, but in the altered carotenoid, these carbon atoms are only bound with ‘single’ bonds. Marshall and Olcott Marshall (2010) describe the Raman spectra β-carotane and lycopane and show us what to look for in the fossil record. These altered carotenoids could be the only traces of a once teeming ecosystem, and we might otherwise never know about them if not for spectroscopy.
Marshall P, Olcott Marshall A. 2010. The potential of Raman spectroscopy for the analysis of diagenetically transformed carotenoids. Philosophical Transactions of the Royal Society A 368: 3137–3144.
Illuminating Fossils 
