Newly discovered fossil evidence of a close relative to all animals (Animalia)

First putative occurrence in the fossil record of choanoflagellates, the sister group of Metazoa

Carolina Fonseca, João Graciano Mendonça Filho, Matías Reolid, Luís V. Duarte, António Donizeti de Oliveira, Jaqueline Torres Souza & Carine Lézin 

Summarized by Jacob Fogg, who  is a student at the University of South Florida studying geology. All his life he has been interested in the natural world and how it works. He came into USF as an undeclared major and after taking an intro environmental science course, he was hooked. Jacob hopes to work in some capacity as a geologist when he graduates from USF in 2023. 

Purpose of Study: Choanoflagellates are single celled organisms that have been found to be the closest living relative to animals that we know of. We know this because sponges, which are a part of the animal kingdom, contain cells called choanocytes that behave in the same manner as choanoflagellates. Despite knowing a great deal about these organisms and their relationship with animals, scientists have never found evidence of them in the fossil record until now. With new fossil evidence of choanoflagellates, scientists can now study the structure and function of these organisms, in relation to how they work in animals, and the environment in which they lived many years ago.

Data Used: 22 rock samples were collected from the Betic Mountain System in southern Spain. These rocks are from the Cretaceous Period, approximately 100 million years ago, when this area was a shallow sea. The samples contained a material called kerogen, which is a form of organic matter that has undergone heat and pressure. If kerogen is continuously exposed to more heat and pressure, then it will eventually become the natural gas that we use in our everyday lives. It was in the kerogen that scientists were able to find the preserved choanoflagellate fossils.

Methods: Researchers crushed up the collected samples and treated it with an acid bath to remove the rock material. Then, they concentrated the kerogen using a heavy liquid and centrifuged the kerogen (meaning, the scientists placed the samples in a machine that spun the kerogen, causing it to separate from any excess water). The kerogen was then placed on slides and examined under a microscope and exposed to white and blue florescent lights to pick out organic particles. It was here where scientists were able to find choanoflagellate fossils. The fossils were then analyzed through the process of confocal laser scanning microscopy (CLSM). This process takes microscopic photos of the samples using high resolution imaging, which can be used to create a 3D model of what you are imaging. CLSM is a very sophisticated imaging process.

Results:  Using the 3D model from CLSM (Fig. 1), scientists were able to directly observe the structure of the choanoflagellates. In the model they created, scientists could see how the choanoflagellates form colonies. The choanoflagellates do this by constructing intercellular bridges and using their filopodia (similar to antennas) to connect their cell bodies together and create a large colony. Scientists were also able to make out the cell bodies of individual choanoflagellates and their flagella (similar to tails). When analyzing the organic material of the rocks, scientists were able to find other microorganisms, such as freshwater microplankton. This suggests that the choanoflagellates in the samples collected were in freshwater environment. Now, in the future, if we find choanoflagellate fossils somewhere in the world, we can infer what type of environment was likely present when the choanoflagellates were living.

Photos A-F look like a hairy ball. The ball is composed of the individual cell bodies of the choanoflagellates which looks like smaller round structures. The hair coming from the large ball is the flagellate of the individual cells. The hairy ball is approximately 60 micrometers in diameter. That’s 0.06 millimeters!
Figure shows the 3D model of choanoflagellates constructed from CLSM. Photo A shows a choanoflagellate colony. They bind together and use their tails (flagella (flagellum singular), the wavy lines that come off of the round structure) to grab food and bring it into the colony. Photos D and F show the cell body of an individual member of the colony.

Why is this study important?  This study is important because choanoflagellates are the link between bacterial organisms and sponges, which are likely among the earliest evolved animals. This means that choanoflagellates are one of the most closely related living things to animals, despite not being animals themselves. . Understanding how these organisms form colonies and sustain themselves can give us insight into how the compare to similar cells that are contained in animal bodies. We can then get a better understanding of how animal cells work.

Broader Implications beyond this study: Now that we have fossilized evidence of choanoflagellates, we have a link between animals and eukaryotic cells. With further research, we may be able to find transitional fossils that show how choanoflagellates became incorporated into animal cells, like in sponges. 

Citation: Fonseca, C., Mendonça Filho, J. G., Reolid, M., Duarte, L. V., de Oliveira, A. D., Souza, J. T., & Lézin, C. (2023). First putative occurrence in the fossil record of choanoflagellates, the sister group of Metazoa. Scientific Reports13, 1242.