When Yosuke Nagahata of the Institute of Evolutionary Biology in Spain realized that blood cells circulating through his body carried a 700-million-year genetic legacy from single-celled ancestors, he felt a jolt of connection to the deep past. His team at Kyoto University had just completed an extraordinary genealogy: a full family tree of blood cell evolution that traces our immune defenses back to the moment multicellular life began.
The discovery matters because it reveals something profound about our bodies. Almost every animal species has blood cells, but their exact composition and lineage have remained largely mysterious to science. Understanding where these cells came from—and how they evolved—opens new windows into how life has learned to protect itself from disease across hundreds of millions of years.
The research, published in the Proceedings of the National Academy of Sciences, began with a technical breakthrough. Hiroshi Kawamoto's team at Kyoto University developed a new analytical method to compare gene expression profiles across different cell lineages and animal species. They didn't stop with comparing humans and mice—they went further back, including unicellular organisms to trace blood cells to their evolutionary origins.
What they found was striking. Among all human blood cell types, macrophages showed the closest resemblance to single-celled organisms. This suggested that when animals first evolved, macrophages were among the first blood cells to emerge. To confirm the timing, the researchers traced the gene FOS—which is expressed in blood cells across virtually all animal species—back to a single-celled ancestor that lived approximately 700 million years ago. This timing lines up precisely with when multicellular animals themselves appeared on Earth.
The family tree that emerged from this analysis is elegant and revealing. Macrophages, those ancestral cells, spawned the first branch: mast cells diverged from them. From mast cells, prototypic T cells and red blood cells subsequently branched off. Meanwhile, prototypic B cells split from macrophages after mast cells had already separated. This branching pattern shows how early animals repurposed genetic material they had inherited from their single-celled ancestors, creating an expanding arsenal of immune defenses as they diversified into thousands of species.
What makes this finding especially moving is its implication: the evolutionary history of the past 700 million years is literally written into the differentiation pathways of our blood cells. Every time our immune system activates, every time new blood cells form, we're watching ancient history unfold in miniature. "The differentiation pathways of vertebrate blood cells reflects the 700-million-year evolutionary history of these cells," Kawamoto reflected on the culmination of the team's work.
Looking forward, researchers believe this new analytical method could help solve medical mysteries. By understanding how blood cells evolved, scientists may gain fresh insights into how cancer develops—essentially seeing aberrant cell behavior through the lens of evolutionary deep time. This perspective could lead to new treatments based on how our cells are supposed to work.
The blood coursing through our veins is not just a delivery system or an immune patrol—it's a living archive, carrying forward the innovations of Earth's earliest life forms into the complex organisms we are today.