The Chicken or the Egg? An Ancient Microbe Says Egg

Chromosphaera perkinsii is a single-celled species discovered in 2017 in marine sediments around Hawaii. The first signs of its presence on Earth have been dated at over a billion years, well before the appearance of the first animals. Now, a team that includes Bigelow Laboratory Senior Research Scientist John Burns has observed that this species forms multicellular structures that bear striking similarities to animal embryos. Their discovery suggests that the genetic instructions responsible for embryonic development were already present before the emergence of animal life — or that C. perkinsii evolved independently to develop similar processes. Nature would therefore have had the genetic tools to “create eggs” long before it “invented chickens.”

“This paper gets at the core question of developmental biology, which is how we go from one cell to a whole animal, by looking at a related process in a weird, microbial relative to animals,” Burns said. “We’re using our emerging understanding of the diversity of microbial life to answer questions about how we got here and how life works in other evolutionary branches.”

The findings were recently published in Nature and covered in Nature News.

The first life forms to appear on Earth were composed of a single cell, such as yeast or bacteria. Later, animals evolved, developing from a single, egg cell to form complex, multicellular beings. This process of embryonic development follows precise stages that are remarkably similar between animal species and could date back to a period well before the appearance of animals. However, the transition from single- to multi-celled organisms is still poorly understood.

Omaya Dudin, an assistant professor of biochemistry at the University of Geneva, led the research. His team focuses on Chromosphaera perkinsii, an ancestral species of protist. This microbe separated from the animal evolutionary line more than a billion years ago, offering valuable insight into the mechanisms that may have led to the development of multicellular organisms.

By observing C. perkinsii, the scientists discovered that these cells, once they have reached their maximum size, divide without growing any further, forming multicellular colonies resembling the early stages of animal embryonic development. These colonies persist for around a third of their life cycle and comprise at least two distinct cell types, a surprising phenomenon for this type of organism.

‘‘Although C. perkinsii is a unicellular species, this behavior shows that multicellular coordination and differentiation processes are already present in the species, well before the first animals appeared on Earth,” Dudin said.

Even more surprisingly, the way these cells divide, and the three-dimensional structure they adopt, is strikingly reminiscent of the early stages of embryonic development in animals. Burns led the analysis of the genetic activity within these colonies, examining the development of C. perkinsii on its own and comparing it the development of other species. That effort revealed intriguing similarities with that observed in animal embryos, which suggests that the genetic programs governing complex multicellular development were already present over a billion years ago.

“Some of the genes that the microbe uses to drive its development, and the general patterns they follow, are similar to what we observe in animals,” Burns said. “These ancient relatives of animals may contain some programs that their direct animal ancestors built on through evolution.”

The authors described the study as a new approach to “comparative embryology,” which gives scientists better tools to understand animal development across evolutionary time. Their discovery could also shed new light on a long-standing scientific debate concerning 600-million-year-old fossils that resemble embryos, and could challenge certain traditional conceptions of multicellularity.

“It's fascinating,” said Marine Olivetta, a laboratory technician at the University of Geneva and lead author of the new study. “A species discovered very recently allows us to go back in time more than a billion years.”

Photo Caption: An expansion microscopy image of the multicellular development of Chromosphaera perkinsii, a close cousin of animals. The membranes are red, and the nuceli within the DNA is blue (Copyright Omaya Dudin).