24-isopropyl cholestane is an organic molecule produced by specific

sponges Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and ar ...

, protists and marine

algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

. The identification of this molecule at high abundances in

Neoproterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...

rocks has been interpreted to reflect the presence of multicellular life prior to the rapid diversification and radiation of life during the Cambrian explosion. In this transitional period at the start of the

Phanerozoic The Phanerozoic is the current and the latest of the four eon (geology), geologic eons in the Earth's geologic time scale, covering the time period from 538.8 million years ago to the present. It is the eon during which abundant animal and ...

, single-celled organisms

evolved Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

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 multicellular life 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 Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine current power * Marine debris * Marine energy * Marine habitats * M ...

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 the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozo ...

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 Steranes constitute a group of alkane organic compounds with a cyclopentane-fused hydrogenated phenanthrene structure as the parent nucleus, and they derived from steroids or sterols via diagenetic and catagenetic degradation, such as hydrogen ...

with

chemical formula 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, such as pare ...

C₃₀H₅₄ and

molecular mass The molecular mass () is the mass of a given molecule, often expressed in units of daltons (Da). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The derived quan ...

414.76 g/mol. The

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

has a

cholestane Cholestane is a saturated tetracyclic steroid biosynthetically derived from triterpenoids. 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, it ...

skeleton with an

isopropyl In organic chemistry, a propyl group 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 ...

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 Demosponges or common sponges are sponges of the class Demospongiae (from + ), the most diverse group in the phylum Porifera which include greater than 90% of all extant sponges with nearly 8,800 species worldwide (according to the World P ...

within the

phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...

Porifera Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a Basal (phylogenetics) , basal clade and a sister taxon of the Eumetazoa , diploblasts. They are sessility (motility) , sessile ...

. Like other molecular fossils, the presence of 24-isopropyl cholestane in rocks may indicate whether demosponge were living in or near the rock's

depositional environment In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will b ...

. High abundances of 24-isopropyl cholestane are identified in the Precambrian rocks from the Hufq supergroup in

Oman Oman, officially the Sultanate of Oman, is a country located on the southeastern coast of the Arabian Peninsula in West Asia and the Middle East. It shares land borders with Saudi Arabia, the United Arab Emirates, and Yemen. Oman’s coastline ...

, suggesting the presence of sponges prior to the Cambrian explosion. 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 ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

, 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, i.e., chemical synthesis occurring in biological contexts, is a term most often referring to multi-step, enzyme-Catalysis, catalyzed processes where chemical substances absorbed as nutrients (or previously converted through biosynthe ...

evolved earlier in the sponges, during the

, 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 Cambrian explosion 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 ...

packstone Under the Dunham classification (Dunham, 1962Dunham, R.J. (1962) Classification of carbonate rocks according to depositional texture. In: Classification of Carbonate Rocks (Ed. W.E. Ham), Am. Assoc. Pet. Geol. Mem., 1, 108–121.) system of limesto ...

s from

South Australia South Australia (commonly abbreviated as SA) is a States and territories of Australia, state in the southern central part of Australia. With a total land area of , it is the fourth-largest of Australia's states and territories by area, which in ...

. Through repeated grinding and

photography Photography is the visual arts, art, application, and practice of creating 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 empl ...

, 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 ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

, 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 Cambrian explosion at ~541 Ma. The presence of

sponge Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and a ...

s before ~540 Ma has profound implications for the evolution of multicellular life and the coupling of the

biosphere The biosphere (), also called the ecosphere (), is the worldwide sum of all ecosystems. It can also be termed the zone of life on the Earth. The biosphere (which is technically a spherical shell) is virtually a closed system with regard to mat ...

climate. Climate change before the Cambrian explosion and the subsequent diversification of life are intricately intertwined with understanding the causes of

Snowball Earth The Snowball Earth is a historical geology, geohistorical hypothesis that proposes that during one or more of Earth's greenhouse and icehouse Earth, icehouse climates, the planet's planetary surface, surface became nearly entirely freezing, fr ...

episodes, the deposition of

Banded Iron Formations Banded iron formations (BIFs; also called banded ironstone formations) 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 and e ...

, 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

and the transition from a

euxinic Euxinia or euxinic conditions occur when water is both wikt:anoxic, anoxic and wikt:sulfidic, 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 strat ...

Canfield ocean The Canfield Ocean model was proposed by Geochemistry, geochemist Donald Canfield to explain the composition of the ocean in the middle to late Proterozoic. History In a paper published in 1998 in ''Nature (journal), Nature'', Canfield argued tha ...

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 pC, 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 t ...

may not provide strong constraints on

O₂ levels.

Caveats

There are several lines of logic against interpreting 24-isopropyl

as a

biomarker In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, ...

for

demosponge Demosponges or common sponges are sponges of the class Demospongiae (from + ), the most diverse group in the phylum Porifera which include greater than 90% of all extant sponges with nearly 8,800 species A species () is often de ...

: # 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 an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce 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 molecular fossils, which plagues many

studies. # 24-isopropyl cholestane can be formed through sedimentary diagenesis of other

organic molecules Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-cont ...

, 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

hopane Hopane is a natural chemical compound classified as a triterpene. It forms the central core of a variety of other chemical compounds which are collectively known as hopanoids. The first compound of the hopane family to be isolated and characte ...

s, 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 (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

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 geochronolo ...

, 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

s before the Cambrian explosion 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 preserve ...

s (although there is tentative evidence for sponge-like structures in the latest-

). Moreover, once fossils of sponges do appear during the

Paleozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...

, 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, have myocytes and are able to move, can reproduce sexually, and grow from a hol ...

diversified and sponges proliferated.

References

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