The blood running through your veins has a 700-million-year-old story, and scientists just worked out how it begins

Every drop of blood in your body is doing something ancient. It is carrying oxygen, fighting infections, clotting wounds, and patrolling for threats, a set of tasks so fundamental to animal life that nearly every creature with a backbone does some version of the same thing. But where did blood cells come from in the first place? How does a single fertilised egg, over hundreds of millions of years of evolution, produce something as complex and specialised as an immune system? Researchers at Kyoto University have now published what may be the most complete answer yet to that question, and the answer is considerably more humbling than anyone expected. Your immune cells, it turns out, are not just descended from ancient animals. They trace their origins to single-celled organisms that lived on this planet 700 million years ago, long before complex animal life existed at all. The legacy of those microscopic ancestors is still circulating inside you, right now, every time your immune system does its job.

What scientists discovered about the origin of blood cells

The team at Kyoto University developed a new analytical method that compared gene expression patterns across many types of cells and animal species simultaneously. Gene expression, which genes are switched on or off in a given cell at a given time, functions like a fingerprint, revealing what a cell does and where it came from. By building evolutionary family trees from these patterns and comparing them across the entire animal kingdom, the researchers were able to trace blood cell lineages backwards through time with a precision that previous methods could not achieve.They also did something no previous study had attempted at this scale: they compared the gene expression of blood cells not just across animals, but with unicellular organisms, the single-celled life forms that preceded complex animals entirely.

How macrophages connect humans to Earth‘s earliest life

Among the human blood cell lineages examined, macrophages showed the strongest similarities to unicellular organisms. This finding suggests that the earliest blood cells may have resembled macrophage immune cells that engulf harmful microbes and cellular debris.Macrophages are, in a sense, the original immune cell. They work by engulfing threats surrounding a harmful microbe or piece of cellular debris, swallowing it, and neutralising it. This is the same basic mechanism that single-celled organisms use to eat. A single-celled organism feeds by wrapping itself around a particle and absorbing it. A macrophage defends the body by doing almost exactly the same thing. The similarity is not coincidental. It is evolutionary.The team also traced the gene FOS, which is widely expressed in blood cells across many animal species, back to a unicellular ancestor that lived about 700 million years ago. This indicates that the first blood cells likely emerged around the same time multicellular animals first appeared on Earth.Seven hundred million years ago, there were no fish, no insects, no plants with roots. The most complex life on Earth was colonies of cells beginning, for the first time, to cooperate. The gene that runs through the blood cells of every vertebrate alive today was already present in the single-celled ancestors of those colonies.

The 700-million-year blood cell family tree

The analysis revealed how different blood cell types may have branched off over time. Mast cells appear to have evolved from macrophages, whilst early versions of T cells and red blood cells later emerged from mast cells. Prototypic B cells branched directly from macrophages after mast cells had already separated.This is a remarkably tidy lineage. The macrophage, the ancient engulfer, the closest living relative of those original single-celled ancestors, sits at the root of the tree. From it, over hundreds of millions of years, the full complexity of the modern immune system branched outward. The T cells that coordinate immune responses. The B cells that produce antibodies. The red blood cells that carry oxygen. All of them, the research suggests, are descendants of the same ancestral line that began with a single cell wrapping itself around something it wanted to eat.The findings suggest that early animals created the first blood cells by reusing genetic material inherited from ancient single-celled ancestors, and that the development pathways of modern blood and immune cells still reflect this 700-million-year evolutionary history.

Why this research matters for cancer and disease

The study is not merely a piece of deep evolutionary history. The researchers believe their new analytical method could be applied to investigating the evolutionary origins of diseases, including cancer. Understanding how blood cells differentiated over 700 million years may illuminate why certain developmental pathways go wrong in leukaemia and other blood cancers, because the pathways that cancer disrupts are the same ancient pathways the study has now mapped.“I feel deeply moved by these findings, which represent the culmination of our work and illustrate that the differentiation pathways of vertebrate blood cells reflect the 700-million-year evolutionary history of these cells,” said team leader Hiroshi Kawamoto. “When I let it sink in that this legacy from so long ago is circulating within my body as blood cells, I feel closer to our distant ancestors,” added first author Yosuke Nagahata.That instinct, the sense of connection across an almost incomprehensible span of time, is perhaps the most honest response to what the study reveals. The immune system you rely on to fight every infection, heal every wound, and neutralise every threat your body encounters is not a modern invention. It is 700 million years old. It was already working, in its earliest form, before animals with eyes, or limbs, or brains existed anywhere on Earth.