24-isopropyl cholestane is an organic molecule produced by specificsponges, protists^[1] and marinealgae.^[2] The identification of this molecule at high abundances inNeoproterozoic rocks has been interpreted to reflect the presence ofmulticellular life prior to the rapid diversification and radiation of life during theCambrian explosion.^[3][4] In this transitional period at the start of thePhanerozoic, single-celled organismsevolved to produce many of the evolutionarylineages present on Earth today.^[5] Interpreting 24-isopropyl cholestane in ancient rocks as indicating the presence ofsponges before this rapid diversification event alters the traditional understanding of the evolution ofmulticellular life and the coupling of biology to changes inend-Neoproterozoic climate. However, there are several arguments against causally linking 24-isopropyl cholestane tosponges based on considerations ofmarine algae and thepotential alteration of organic molecules over geologic time.^[6] In particular the discovery of 24-isopropyl cholestane in rhizarian protists implies that this biomarker cannot be used on its own to trace sponges.^[1] Interpreting the presence of 24-isopropyl cholestane in the context of changingglobal biogeochemical cycles at theProterozoic-Phanerozoic transition remains an area of active research.

24-isopropyl cholestane (figure 1, left) is a C₃₀sterane withchemical formula C₃₀H₅₄ andmolecular mass 414.76 g/mol. Themolecule has acholestane skeleton with anisopropyl moiety at C₂₄ and is the geologically stable form of 24-isopropyl cholesterol.^[3] A related and important molecule is 24-n-propyl cholestane (figure 1, right), also with the cholestane skeleton, but with ann-propyl moiety at C₂₄.

24-isopropyl cholestane is produced copiously by a particular group ofsponges in the classDemospongiae within thephylumPorifera.^[2][7] Like othermolecular fossils, the presence of 24-isopropyl cholestane in rocks may indicate whether demosponge were living in or near the rock'sdepositional environment. High abundances of 24-isopropyl cholestane are identified in thePrecambrian rocks from the Hufq supergroup inOman, suggesting the presence of sponges prior to theCambrian explosion.^[3] However, sponges are not the only organisms that produce 24-isopropyl cholestane, so the identification of this biomarker is not uniquely linked to the presence of demosponge.

While marinepelagophyte algae predominantly produce 24-n-propylcholestane,^[8] they also produce 24-isopropyl cholestane. The two possible sources of 24-isopropyl cholestane to rocks, the demosponge and the algae, can be decoupled by considering the ratio of 24-isopropyl cholestane to 24-n-propyl cholestane. In many rocks, this ratio is 0.2-0.3.^[3] However, in rocks fromOman, the ratio of steranes is 0.52-16.1, with an average value of 1.51, which strongly suggests input of sponge organic matter.^[3] Notably, these elevated values disappear during theCambrian, and the ratio of 24-isopropyl cholestane to 24-n-propyl cholestane is used an age-specific proxy for theProterozoic-Phanerozoic transition.^[9]

Recent research in molecular clocks has argued that the ability to produce 24-isopropyl cholesterol evolved independently in both the demosponge and algae.^[4] However, it appears that thebiosynthesis evolved earlier in the sponges, during theNeoproterozoic, and that the ability to perform thebiosynthesis was not present in algae until thePhanerozoic. If correct, these results would give scientists much more confidence in interpreting elevated levels of 24-isopropyl cholestane in ancient rocks as reflecting the presence of sponges.

Additional evidence for sponge evolution before theCambrian explosion is found inbioclasticpackstones fromSouth Australia.^[10] Through repeated grinding andphotography, researchers constructed 3D models ofasymmetric structures with ~1 mm-diameter interconnected channels contained within this rock. The complex network of tunnels appears inconsistent withfungi oralgae, and the researchers tentatively suggested that they are primitive sponges. This interpretation is controversial because the structures pre-date the first appearance ofother sponge fossils and the structures are only known to occur within a singlesedimentary sequence.

While Loveet al. (2009) argues for the presence of sponges in rocks below the Marinoan cap carbonate at ~635 Ma (millions of years ago),^[3] Antcliffe (2013) estimates the age of the biomarker-bearing rock to be between 645 Ma and ~580 Ma.^[6] Most recently, Goldet al. (2016) writes that the age of rocks containing 24-isoproylcholestane have an age between ~650 Ma and 540 Ma.^[4] In all cases, estimates agree that the age of the rocks containing 24-isoproylcholestane pre-date theCambrian explosion at ~541 Ma.

The presence ofsponges before ~540 Ma has profound implications for the evolution of multicellular life and the coupling of thebiosphere toNeoproterozoic climate. Climate change before theCambrian explosion and the subsequent diversification of life are intricately intertwined with understanding the causes ofSnowball Earth episodes,^[11] the deposition ofBanded Iron Formations,^[12] and the second step in therise of atmospheric oxygen.^[13] In particular, the presence of sponges raises questions of the minimum dissolved O₂ content of the oceans in the lateNeoproterozoic and the transition from aeuxinicCanfield ocean to the modern oxygenated deep-ocean. However, sponges appear to require very little O₂ to survive, so their presence in thePrecambrian may not provide strong constraints onProterozoic O₂ levels.^[14]

There are several lines of logic against interpreting 24-isopropylcholestane as abiomarker fordemosponge:^[6]

1. Much of the argument forPrecambrian sponges is grounded in the observation thatpelagophyte algae produce organic matter with a low ratio of 24-isopropyl cholestane to 24-n-propyl cholestane, but that this ratio is high in ancient rocks. However, the observed change in thesterane ratio could also be explained ifalgae changed the relative abundances in which they producesteranes over the past 600 million years. In a similar line of argument, it is possible that anotherextinct organism from which the algae descended produced organic matter with a higher ratio of 24-isopropyl cholestane to 24-n-propyl cholestane. As argued above, recent evidence has suggested that the algae's synthesis pathway only arose during thePhanerozoic,^[4] which tempers this argument. More generally, these concerns address the issue of insufficient specificity inmolecular fossils, which plagues manybiomarker studies.
2. 24-isopropyl cholestane can be formed throughsedimentary diagenesis of otherorganic molecules, so the high ratio of 24-isopropyl cholestane to 24-n-propyl cholestane could simply reflect thepost-depositional transformation of organic matter. Some research has addressed this concern by showing a lack of alteration in other organic molecules,^[3] such ashopanes, but subsequent analysis has questioned whether the molecules with minimal alteration could have been contamination from modernpetroleum-derived oil.^[6]
3. There may have been a group ofbacteria living symbiotically with the sponges that also produced 24-isopropyl cholestane.^[15] If thesebacteria produced the biomarker throughoutgeologic time, its presence would not be strictly indicative ofdemosponge. However, as with the marine algae, analysis of the ratio of 24-isopropyl cholestane to 24-n-propyl cholestane may clarify the source of the compounds.
4. It is strange to find spongebiomarkers before theCambrian explosion without accompanying spongefossils (although there is tentative evidence for sponge-like structures in the latest-Neoproterozoic^[10]). Moreover, once fossils of sponges do appear during thePaleozoic, the ratio of 24-isopropyl cholestane to 24-n-propylcholestane returns to its background value. This is surprising because we might expect the ratio to stay elevated or even to increase asmetazoa diversified and sponges proliferated.

• Biomarkers
• Cholestanes
• Sponge biology

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