Creationism in Crisis - Chemical Fossils Show How Life Evolved Over A Billion Years Before 'Creation Week'

Present-day annelid worms, like earthworms and this bearded fireworm, have retained a gene to make some types of lipids that most animals have lost.

Credit: Johnandersonphoto
via Getty Images

Fig. 6: Summary of smt gene loss in the animals.
A simplified animal phylogeny, with the hypothesized presence of smt visualized with blue lines. This tree provides a conservative estimate of the number of smt losses, as it excludes many understudied animal phyla that may also lack the protein.

Brunoir, T., Mulligan, C., Sistiaga, A. et al. (2023)

Molecular Fossils Shed Light on Ancient Life | UC Davis

Early organisms, particularly from before animals with hard body parts like teeth, bones and hard exoskeletons had evolved, leave few traces in the fossil record, but that's not to say they leave no trace whatsoever. What they leave is a chemical signature in the rocks that can last for hundreds of millions, even billions of years.

Sterol lipids, for example are highly stable chemicals that come from cell membranes and can be found in rocks dated to 1.6 billion years old. Since they can only be produced by living organisms, they are compelling evidence for the existence of life when those rocks were laid down.

In the present day, most animals use cholesterol — sterols with 27 carbon atoms (C₂₇) — in their cell membranes. In contrast, fungi typically use C₂₈ sterols, while plants and green algae produce C₂₉ sterols. The C₂₈ and C₂₉ sterols are also known as phytosterols.

C₂₇ sterols have been found in rocks 850 million years old, while C₂₈ and C₂₉ traces appear about 200 million years later. This is thought to reflect the increasing diversity of life at this time and the evolution of the first fungi and green algae.

Early organisms needed to synthesise their own sterols and did so using a gene called smt, but, as more sources of sterols became available by eating fungi and algae, so this gene became redundant and was eventually lost from many evolutionary lines. When this gene disappeared from these lines shows when they began consuming these new sources of sterols.

By constructing a family tree for this gene using data from first annelids then across animal life in general, the UCDavid team were able to map when this gene was lost onto changes in the sterol record in rocks - and they mapped closely to the chemical record in the rocks.

According to a news release from the University of California Davis (UCDavis), where David Gold, associate professor in the Department of Earth and Planetary works in this new field of molecular paleontology, using the tools of both geology and biology to study the evolution of life:

Most animals are not able to make phytosterols themselves, but they can obtain them by eating plants or fungi. Recently, it was discovered that annelids (segmented worms, a group that includes the common earthworm) have a gene called smt, which is required to make longer-chain sterols. By looking at smt genes from different animals, Gold and colleagues created a family tree for smt first within the annelids, then across animal life in general. They found that the gene originated very far back in the evolution of the first animals, and then went through rapid changes around the same time that phytosterols appeared in the rock record. Subsequently, most lineages of animals lost the smt gene.

Our interpretation is that these phytosterol molecular fossils record the rise of algae in ancient oceans, and that animals abandoned phytosterol production when they could easily obtain it from this increasingly abundant food source. If we’re right, then the history of the smt gene chronicles a change in animal feeding strategies early in their evolution.

Professor D. A. Gold, Senior author
Department of Earth and Planetary Sciences
University of California, Davis, Davis, CA, USA>

The team's findings are published open access in Nature Communications:

Abstract

Steranes preserved in sedimentary rocks serve as molecular fossils, which are thought to record the expansion of eukaryote life through the Neoproterozoic Era ( ~ 1000-541 Ma). Scientists hypothesize that ancient C₂₇ steranes originated from cholesterol, the major sterol produced by living red algae and animals. Similarly, C₂₈ and C₂₉ steranes are thought to be derived from the sterols of prehistoric fungi, green algae, and other microbial eukaryotes. However, recent work on annelid worms–an advanced group of eumetazoan animals–shows that they are also capable of producing C₂₈ and C₂₉ sterols. In this paper, we explore the evolutionary history of the 24-C sterol methyltransferase (smt) gene in animals, which is required to make C₂₈₊ sterols. We find evidence that the smt gene was vertically inherited through animals, suggesting early eumetazoans were capable of C₂₈₊ sterol synthesis. Our molecular clock of the animal smt gene demonstrates that its diversification coincides with the rise of C₂₈ and C₂₉ steranes in the Neoproterozoic. This study supports the hypothesis that early eumetazoans were capable of making C₂₈₊ sterols and that many animal lineages independently abandoned its biosynthesis around the end-Neoproterozoic, coinciding with the rise of abundant eukaryotic prey.

Introduction

Organic compounds preserved in rocks–known as molecular fossils or biomarkers–offer a unique window into the early evolution of life. Compared to other biological molecules, such as nucleic acids and proteins, lipids are particularly resistant to degradation, with structural features that can be preserved in the geologic record for hundreds of millions, potentially billions, of years. Despite the ever-present risks of contamination and diagenetic alteration^1,2,3 the biomarker field is coalescing around best practices, and clear patterns are emerging4. Steranes, the diagenetic remains of sterol lipids found in eukaryotic cell membranes, have proven particularly informative in Neoproterozoic-age rocks (~1000-541 Ma), where animal fossils are vexingly scarce (Fig. 1). Sterols are present in all eukaryotes and perform essential functions within the cell membrane. In eumetazoan animals and multicellular red algae, these functions are generally performed by the 27-carbon (C₂₇) sterol, cholesterol, while C₂₈, sterols are the dominant sterols in most fungi, and C₂₉ sterols are common in green algae and plants^5,6. These sterols are observed widely throughout the eukaryotic tree of life, suggesting their presence in the last common ancestor⁷. In the geologic record, early proto-steranes have been identified in the Barney Creek Formation from ~1640 Ma, and can be found in rocks until ~850 Ma^8,9. C₂₇ steranes–the diagenetic products of cholesterol–become abundant in Neoproterozoic-age rocks starting around 850 Ma, while C₂₈ and C₂₉ steranes become prominent in the interglacial period ~663–635 Ma4,10. The first appearance of steranes is hypothesized to represent the ecological expansion of early eukaryotes (possibly red algae), with fungi and green algae expanding substantially around 663 Ma^4,10. The C30 sterane isopropylcholestane also occurs around this time and could represent a biomarker for sponges or rhizarian protists^11,12,13. Recently, steranes and bacterial triterpenoids have also been used to taxonomically constrain enigmatic fossils from the Neoproterozoic, including the assignment of Beltanelliformis as a colonial cyanobacterium and Dickinsonia as an animal^14,15,16. Taken together, these steranes paint a picture of increasing eukaryote diversity leading up to the Cambrian (~541–485 Ma) radiation of fossils…

Fig. 1: Overview of the Neoproterozoic sterane record.

The geologic timescale is provided on the left, with dates in hundreds of millions of years. Arrows indicate the approximate time when various biomarkers become detectable above background thresholds. The site where sterols are normally methylated by the gene 24-C sterol methyltransferase (carbon-24) is noted on ergostane with an arrow.

Brunoir, T., Mulligan, C., Sistiaga, A. et al.
Common origin of sterol biosynthesis points to a feeding strategy shift in Neoproterozoic animals.
Nat Commun 14, 7941 (2023). https://doi.org/10.1038/s41467-023-43545-z

Copyright: © 2023 The authors.
Published by Springer Nature Ltd. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

In summary, the fossil record is now pushed back to well before the Cambrian radiation in the form of highly stable chemical records in rocks. These show not only when the major taxons of eukaryote life evolved but when they evolved in response to proliferating organisms that synthesized sterols by consuming then and no longer needed to synthesise their own sterols.

And so our knowledge of what was happening to life on Earth and how it evolved is pushed back even further into that long expanse of 'pre-Creation' history.

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