Why All Life on Earth is Made of Cells

“With evolutionary biology, it’s not best practice to come up with a reason why something might have occurred and then declare that’s why it did occur,” he says. “And when you’re studying the transition between non-life and life, there is no definition in the literature that says life is made out of cells.”

Determining when and how all life on Earth came to be made of cells is a way to approach the tantalizing question of the origin of life. But Goldman’s research zeroes in on the insights gleaned from examining how this fundamental biological process of cellularity emerged.

For example, he’s conducted studies involving computer simulations that model Earth’s early environment and the organisms that evolved within it. These digital life simulations allowed student researchers in his lab at Oberlin to interrogate specific evolutionary pressures and dynamics related to the origin of cellularity. Their findings were published in a 2020 article in the Journal of Molecular Evolution, with students Diep H. Nguyen ’19 and Yuta A. Takagi ’16 as co-first authors.

The link between cellularity and proteins

Goldman’s most recent research on cellularity, published in July 2024 in Journal of Molecular Evolution, investigated the tree of life—a model depicting how all life is related, with new branches emerging as new traits evolve— to uncover when key features of cellularity evolved early on. 

The root of the tree of life branches to form two “domains,” the bacteria (think single cells without a nucleus or other internal cellular organization, like E. coli) and the archaea (also single cellular life, but with some specific proteins and ribosomal features that make them distinct from bacteria). Our form of life, the eukaryotes–which includes humans, plants, slugs, or life with both a nucleus and internal cell organization–emerged from the archaeal group.

Because bacteria, archaea, and eukaryotic life are all very different, some researchers have wondered if cellularity evolved after the last universal common ancestor of all life (LUCA) that preceded them. But other researchers have suggested that cellularity occurred earlier than the LUCA—even during the origin of life itself. For example, Goldman points out that if you take the lipid portion that was part of early meteorites, the fats will form a bubble structure that looks like a cell membrane. These types of early, naturally occurring membranes could have been precursors to cellularity as we know it today. 

In order to build a better timeline of the origin of cellularity, Goldman and the student researchers in his lab reconstructed the functions proteins present in three things: the LUCA; the last archeal common ancestor; and the last bacterial common ancestor. Together, these represent the first branch on the tree of life. They found that a number of proteins involved in cellular membranes were present in all three reconstructions, despite the variations between bacterial and archeal life. This indicates that cellularity evolved before the archaeal and bacterial branches split—and perhaps before the LUCA. 

“Organisms across the tree of life use slightly different chemical components to build their membranes, and this has been taken as evidence that the last universal common ancestor of life was not cellular,” Goldman says. “But this study shows that cellularity was well established by the time of the last universal common ancestor of life. Organisms at that point were already controlling what was getting through their cellular membrane, using their cellular membrane to generate energy through an electron transport chain, and directing the process of reproduction through cell division.” 

Currently, the members of Goldman’s lab are working on releasing an update of “LUCApedia,” a database of the available information about early life and its biological functions, to enable more researchers to investigate aspects of early evolutionary history. “We won’t really be able to know everything about early evolution, because we don’t fully understand the complete diversity of life on earth,” Goldman says. “But looking at evolutionary history gives us insight into why biology works the way it does—and as diverse as the biosphere is today, it gives us an understanding of what all life shares.”