Lost world of complex life and the late rise of the eukaryotic crown
By: Jochen J. Brocks, Benjamin J. Nettersheim, Pierre Adam, Philippe Schaeffer, Amber J. M. Jarrett, Nur Güneli, Tharika Liyanage, Lennart M. van Maldegem, Christian Hallmann & Janet M. Hope
Summarized by: Neha Tengshe is a junior studying biological sciences at Binghamton University. She loves video games and hanging out with her friends. She’s super interested in phylogeny (the study of how living things are related), communication systems in animals and plants, and biology as a whole!! She loves her family and working on wikis for her interests, though she finds code frustrating at times. She’s excited to be able to share this article about one of her passions with the readers!!
What was the hypothesis being tested? The point of the paper was to examine and potentially explain the contrast between the theoretical timeline for the emergence of multicellular organisms (eukaryotes) versus the tangible evidence of their appearance in the fossil record. The theoretical timeline was produced by using a molecular clock model. Molecular clocks are a mathematical model that count how many genetic mutations have accumulated between organisms and estimate a rate at which the mutations appeared, which can provide an estimate of when different organisms first evolved. Molecular clock models suggest that eukaryotes emerged earlier than fossil remains can be found, so the researchers decided to try a different approach: they looked for biomarkers, or chemical markers, that proto-eukaryotes might have left behind in the rock record to figure out when they began their reign. The chemicals they chose to investigate are called sterols. Sterols in eukaryotes are used to regulate cell membranes, so they’re both vital to survival and generally abundant, as long as the eukaryotes that synthesize them are also abundant. Meanwhile, bacteria (which are prokaryotes, not eukaryotes) use a different chemical for the same purpose as sterols. The chemicals bacteria produce are called hopanepolyols, and the change from the simpler hopanepolyols to the sterols the crown group uses are an important indicator of the evolutionary timeline of eukaryotes. Specifically, the researchers decided to compare the number of fossilized hopanepolyols (which would be preserved as hopanes) to the number of fossilized sterols (which preserve as steranes).
What data was used? The data used are rocks from the Proterozoic Eon, specifically around the Tonian Period (around 1 billion to 720 million years ago). The researchers were looking for traces of sterol groups that predated the types of sterol groups that modern-day eukaryotes (like humans) use. In this paper, modern day eukaryotes’ sterol groups are called “Crown-Sterols” (which include mushrooms to bananas to humans to dragonflies, anything with more than one cell alive today is a Crown-Sterol!), since they are the crown group of the eukaryotic line. A crown group is all living relatives and descendants of an ancestor, so for this study, all living eukaryotes act as the “crown group.”
Methods: The researchers used rocks from before the Tonian Period, ground them down into powder, and used various solvents to clean them of potential contaminants. They used this powder to perform mass spectrometry. They heated up the powder until it vaporized and put the gaseous form in a spectrometer, which produced a graph with peaks that are unique to certain substances. Essentially, different chemicals have specific signatures that they leave when being analyzed with a mass spectrometer. They then matched the peaks to known formulas to identify the target compounds (i.e. the steranes and the hopanes).
Results: Researchers found that there was another group of eukaryotes that functioned on simple sterols (Ursterols), due to traces of these simpler sterols (the traces are called Ursteroids) being preserved in the researched rocks. Researcher Konrad Bloch, who deciphered the cholesterol pathway, suggested that the full pathway for cholesterol emerged from other pathways for complete but simpler sterols (which he called Ursterols), and the synthesis yielded more and more complicated products (Cholesterol) over time because they performed better than their simpler predecessors (Ursterols). The discovery of the ursterol-using eukaryotes (Ur-Eukaryotes) helps realign the timings inferred by theoretical models like the molecular clock with what living groups can be observed today. The researchers argued that Crown-group eukaryotes co-existed with this group of Ur-Eukaryotes, until the Tonian when the Crown group’s more complex sterol eventually won out.
Figure 2 is a family tree diagram. There are seven branches that end in nothing. One branch is labeled LECA, which then splits into 8 branches. The first is labeled Alveolates, the second is labeled Rhizaria, the third is labeled Haptophytes, the fourth is labeled Chlorophytes and is green, the fifth is labeled Rhodophytes and is red, the sixth is labeled Holozoa, the seventh is labeled fungi, and the last is Amebozoa. There is a circle at the start of the Rhizaria split, which connects to an image of the oldest known Rhizaria in Figure 3. There is a green circle that connects to an image of the oldest known chlorophyte in Figure 3, and a green triangle on the chlorophyte line, labeled “Rise of Chlorophytes”. There is a red circle on the Rhodophyte line, which connects to an image of the oldest Rhodophytes, and a triangle labeled “Rise of Rhodophytes”. On the Holozoa line, there is a circle and triangle one after another. The circle is labeled “Ediacaran biota”, and the triangle is labeled “Cambrian Explosion”. There is a circle on the Fungi line, which connects to an image of the “Oldest fungus”. There is a circle on the Amoebozoa line at the exact same plane as the circle on the Rhizaria line, and the circle on the Amoebozoa line is labeled “oldest Amebozoan”.
Figure 3 is pictures of various fossils, connected to Figure 2. The oldest known Rhizaria looks like a sponge. The oldest known chlorophyte looks like a bunch of noodles. The oldest known rhodophyte looks like a lavender flower, but brown. The oldest known fungi looks like a circle with a worm coming out of it. The oldest known amoebozoa looks like a human stomach, or a minecraft squid ink sac.
Why is this study important? Having a tighter range on when eukaryotes emerged gives researchers a better understanding of the timeline for how complex life on earth evolved. Knowing this gives researchers a guideline of what age of rocks they should investigate for more clues. It also gives them a reliable way to measure how far the shifts of diversity and abundance between prokaryotes and eukaryotes has come, just by measuring the ratio of hopanes to steranes. This paper opens the door to tracking one of paleontology’s biggest challenges (due to lack of preserved fossils, as these earliest fossils didn’t have bones or teeth, just soft tisue), the rise of eukaryotic life. It also finds evidence of another, previously theorized but undetected group of eukaryotes, the multicellular organisms that rule the Earth today.
Broader Implications beyond this study: One of the fundamentals of biology is cell theory, which posits that all cells came from other cells. That means that everything is related, and if you go back far enough, you will find an organism that all eukaryotes descended from, what scientists call “the Last Eukaryotic Common Ancestor” (LECA, for short). Discovering details about LECA helps to explain the origin of many modern eukaryotic systems and complex life as a whole.
Citation: Brocks, J.J., Nettersheim, B.J., Adam, P., Schaeffer, P., Jarrett, A.J.M., Güneli, N., Liyanage, T., van Maldegem, L.M., Hallmann, C., Hope, J.M. Lost world of complex life and the late rise of the eukaryotic crown. Nature 618, 767–773 (2023). https://doi.org/10.1038/s41586-023-06170-w