The classic question, “What came first, the egg or the chicken?” has intrigued philosophers and scientists for centuries. A recent discovery, however, may finally provide some clarity on this ancient dilemma. In 2017, researchers identified a unicellular species known as Chromosphaera perkinsii, found in oceanic sediments near Hawaii, which could hold the answer.

Chromosphaera perkinsii (C. perkinsii), a single-celled protist, is estimated to have existed over a billion years ago, predating the first animals by a considerable margin. Scientists from the University of Geneva (UNIGE) recently observed that C. perkinsii can form multicellular structures with a striking resemblance to early animal embryos. This behavior, which involves cells dividing and organizing into specific shapes, suggests that the genetic foundations for multicellular development might have existed long before animals themselves appeared.

Omaya Dudin, a lead researcher on this study, explains that C. perkinsii separates from the animal evolutionary line by over a billion years. Nonetheless, this species can form small colonies that mirror early embryonic stages in animals. After reaching a certain size, the cells divide without further growth, forming multicellular colonies that persist for a significant part of their life cycle. These colonies even contain at least two distinct cell types, a surprising level of complexity for a unicellular organism.

Celula gallina huevo
A cell of the ichthyosporean C. perkinsii showing distinct signs of polarity, with clear cortical localization of the nucleus before the first cleavage. Microtubules are shown in magenta, DNA in blue, and the nuclear envelope in yellow. Credit: DudinLab / UNIGE

The fact that C. perkinsii exhibits this level of cell differentiation and organization raises the possibility that the genetic blueprint for embryonic development predated the emergence of animals. This suggests that nature might have developed the tools to “create eggs” — or the mechanisms for forming complex, multicellular structures — long before it “invented chickens”, or animals as we know them.

In collaboration with Dr. John Burns from the Bigelow Laboratory for Ocean Sciences, researchers analyzed the genetic activity within these colonies. They discovered a striking resemblance between the genetic programs of C. perkinsii and those seen in animal embryos. This insight points to a remarkable possibility: the genetic mechanisms behind complex multicellularity might be far older than we previously thought, dating back to a time when only single-celled organisms populated the Earth.

This discovery adds a new layer to our understanding of life’s evolution on Earth. It suggests that the roots of multicellular life may not lie solely with the first animals but could be traced back to single-celled organisms like C. perkinsii, which exhibited early forms of cellular cooperation and organization. This raises intriguing questions about how and why these complex genetic programs developed in such ancient organisms.

Huevo gallina celula
Images of the multicellular development of the ichthyosporean Chromosphaera perkinsii, a close cousin of animals. In red we can see the membranes and in blue the nuclei with their DNA. The image was obtained using expansion microscopy. Credit: O. Dudin / UNIGE

Moreover, these findings could help address a longstanding debate over 600-million-year-old fossils that resemble embryos. If the genetic foundations for multicellular development existed well before animals, it might mean that these ancient fossils are not necessarily animal embryos but rather the remains of similar single-celled organisms that had already evolved the capacity for multicellular structures.

In sum, the research on C. perkinsii provides a fascinating perspective on the age-old question of “Which came first, the egg or the chicken?” The answer might be that the mechanisms for forming an “egg” existed long before animals, suggesting that nature had already set the stage for multicellular development, awaiting only the arrival of organisms complex enough to become animals as we know them today.

SOURCES

Université de Genève

Olivetta, M., Bhickta, C., Chiaruttini, N. et al. A multicellular developmental program in a close animal relative. Nature (2024). doi.org/10.1038/s41586-024-08115-3

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