24-isopropyl cholestane is an organic molecule produced by specific

sponges Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through ...

, protists and marine

algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...

. The identification of this molecule at high abundances in Neoproterozoic rocks has been interpreted to reflect the presence of

multicellular life A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni ...

prior to the rapid diversification and radiation of life during the

Cambrian explosion The Cambrian explosion, Cambrian radiation, Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately in the Cambrian Period when practically all major animal phyla started appearing in the fossil recor ...

. In this transitional period at the start of the

Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 538.8 million years to the present, and it began with the Cambrian Period, when anima ...

, single-celled organisms

evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation t ...

to produce many of the evolutionary lineages present on Earth today. Interpreting 24-isopropyl cholestane in ancient rocks as indicating the presence of

before this rapid diversification event alters the traditional understanding of the evolution of

and the coupling of biology to changes in end-Neoproterozoic climate. However, there are several arguments against causally linking 24-isopropyl cholestane to

based on considerations of marine algae and the potential alteration of organic molecules over geologic time. In particular the discovery of 24-isopropyl cholestane in rhizarian protists implies that this biomarker cannot be used on its own to trace sponges. Interpreting the presence of 24-isopropyl cholestane in the context of changing global biogeochemical cycles at the

Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...

transition remains an area of active research.

24-isopropyl cholestane

Chemical argument for Precambrian sponges

24-isopropyl cholestane (figure 1, left) is a C₃₀

sterane Sterane (cyclopentanoperhydrophenanthrenes) compounds are a class of tetracyclic compounds derived from steroids or sterols via diagenetic and catagenetic degradation and saturation. Steranes have an androstane skeleton with a side chain at carb ...

with

chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

C₃₀H₅₄ and

molecular mass The molecular mass (''m'') is the mass of a given molecule: it is measured in daltons (Da or u). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The related quanti ...

414.76 g/mol. The

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

has a

cholestane Cholestane is a saturated tetracyclic triterpene. This 27-carbon biomarker is produced by diagenesis of cholesterol and is one of the most abundant biomarkers in the rock record. Presence of cholestane, its derivatives and related chemical compoun ...

skeleton with an

isopropyl In organic chemistry, propyl is a three-carbon alkyl substituent with chemical formula for the linear form. This substituent form is obtained by removing one hydrogen atom attached to the terminal carbon of propane. A propyl substituent is ofte ...

moiety at C₂₄ and is the geologically stable form of 24-isopropyl cholesterol. A related and important molecule is 24-n-propyl cholestane (figure 1, right), also with the cholestane skeleton, but with an n-propyl moiety at C₂₄. 24-isopropyl cholestane is produced copiously by a particular group of

in the class Demospongiae within the

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...

Porifera Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through th ...

. Like other

molecular fossils A biosignature (sometimes called chemical fossil or molecular fossil) is any substance – such as an element, isotope, or molecule – or phenomenon that provides scientific evidence of past or present life. Measurable attribute ...

, the presence of 24-isopropyl cholestane in rocks may indicate whether demosponge were living in or near the rock's depositional environment. High abundances of 24-isopropyl cholestane are identified in the Precambrian rocks from the Hufq supergroup in

Oman Oman ( ; ar, عُمَان ' ), officially the Sultanate of Oman ( ar, سلْطنةُ عُمان ), is an Arabian country located in southwestern Asia. It is situated on the southeastern coast of the Arabian Peninsula, and spans the mouth of t ...

, suggesting the presence of sponges prior to the

. 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 marine pelagophyte algae predominantly produce 24-n-propylcholestane, 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. However, in rocks from

, the ratio of steranes is 0.52-16.1, with an average value of 1.51, which strongly suggests input of sponge organic matter. Notably, these elevated values disappear during the

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

, and the ratio of 24-isopropyl cholestane to 24-n-propyl cholestane is used an age-specific proxy for the

transition. Recent research in molecular clocks has argued that the ability to produce 24-isopropyl cholesterol evolved independently in both the demosponge and algae. However, it appears that the

biosynthesis Biosynthesis is a multi-step, enzyme-catalyzed process where substrates are converted into more complex products in living organisms. In biosynthesis, simple compounds are modified, converted into other compounds, or joined to form macromolecules. ...

evolved earlier in the sponges, during the Neoproterozoic, and that the ability to perform the

was not present in algae until the

. 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.

Observations of Precambrian sponges

Additional evidence for sponge evolution before the

is found in

bioclast Bioclasts are skeletal fossil fragments of once living marine or land organisms that are found in sedimentary rocks laid down in a marine environment—especially limestone varieties around the globe. some of which take on distinct textures and col ...

ic packstones from

South Australia South Australia (commonly abbreviated as SA) is a state in the southern central part of Australia. It covers some of the most arid parts of the country. With a total land area of , it is the fourth-largest of Australia's states and territories ...

. Through repeated grinding and

photography Photography is the art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employed ...

, researchers constructed 3D models of asymmetric structures with ~1 mm-diameter interconnected channels contained within this rock. The complex network of tunnels appears inconsistent with

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

, and the researchers tentatively suggested that they are primitive sponges. This interpretation is controversial because the structures pre-date the first appearance of other sponge fossils and the structures are only known to occur within a single sedimentary sequence.

Implications

While Love ''et al.'' (2009) argues for the presence of sponges in rocks below the Marinoan cap carbonate at ~635 Ma (millions of years ago), Antcliffe (2013) estimates the age of the biomarker-bearing rock to be between 645 Ma and ~580 Ma. Most recently, Gold ''et al.'' (2016) writes that the age of rocks containing 24-isoproylcholestane have an age between ~650 Ma and 540 Ma. In all cases, estimates agree that the age of the rocks containing 24-isoproylcholestane pre-date the

at ~541 Ma. The presence of

sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...

s before ~540 Ma has profound implications for the evolution of multicellular life and the coupling of the biosphere to Neoproterozoic climate. Climate change before the

and the subsequent diversification of life are intricately intertwined with understanding the causes of

Snowball Earth The Snowball Earth hypothesis proposes that, during one or more of Earth's Greenhouse and icehouse Earth, icehouse Climate, climates, the Earth's surface, planet's surface became entirely or nearly entirely Freezing, frozen. It is believed that ...

episodes, the deposition of

Banded Iron Formations Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness a ...

, and the second step in the rise of atmospheric oxygen. In particular, the presence of sponges raises questions of the minimum dissolved O₂ content of the oceans in the late Neoproterozoic and the transition from a

euxinic Euxinia or euxinic conditions occur when water is both anoxic and sulfidic. This means that there is no oxygen (O2) and a raised level of free hydrogen sulfide (H2S). Euxinic bodies of water are frequently strongly stratified, have an oxic, highly ...

Canfield ocean The Canfield Ocean model was proposed by geochemist Donald Canfield to explain the composition of the ocean in the middle to late Proterozoic. In a paper published in 1998 in ''Nature'', Canfield argued that the ocean was anoxic and sulfidic dur ...

to the modern oxygenated deep-ocean. However, sponges appear to require very little O₂ to survive, so their presence in the

Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...

may not provide strong constraints on

O₂ levels.

Caveats

There are several lines of logic against interpreting 24-isopropyl

as a biomarker for

demosponge Demosponges (Demospongiae) are the most diverse class in the phylum Porifera. They include 76.2% of all species of sponges with nearly 8,800 species worldwide (World Porifera Database). They are sponges with a soft body that covers a har ...

: # Much of the argument for

sponges is grounded in the observation that pelagophyte 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 the

ratio could also be explained if

changed the relative abundances in which they produce

s over the past 600 million years. In a similar line of argument, it is possible that another

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

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 the

, which tempers this argument. More generally, these concerns address the issue of insufficient specificity in

, which plagues many biomarker studies. # 24-isopropyl cholestane can be formed through sedimentary diagenesis of other

organic molecules In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...

, so the high ratio of 24-isopropyl cholestane to 24-n-propyl cholestane could simply reflect the post-depositional transformation of organic matter. Some research has addressed this concern by showing a lack of alteration in other organic molecules, such as hopanes, but subsequent analysis has questioned whether the molecules with minimal alteration could have been contamination from modern petroleum-derived oil. # There may have been a group of

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

living symbiotically with the sponges that also produced 24-isopropyl cholestane. If these

produced the biomarker throughout

geologic time The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochrono ...

, its presence would not be strictly indicative of

. 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. # It is strange to find sponge biomarkers before the

without accompanying sponge

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s (although there is tentative evidence for sponge-like structures in the latest- Neoproterozoic). Moreover, once fossils of sponges do appear during the

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

, 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 as

metazoa Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

diversified and sponges proliferated.

References

{{DEFAULTSORT:Isopropylcholestane, 24- Biomarkers Cholestanes Sponge biology