Every second, millions of blood cells travel through your body, carrying oxygen, fighting infections, and helping keep you alive.
But according to a new study, these cells have a much older story than anyone might imagine—one that stretches back about 700 million years to some of Earth’s earliest life forms.
Researchers from Kyoto University wanted to understand where blood cells came from and how they evolved over time.
Although scientists know a great deal about modern blood cells in humans and mice, their ancient origins have remained a mystery.
To investigate, the research team developed a new method for comparing the activity of genes in different cell types across many animal species.
By examining which genes were switched on and off in various cells, the researchers were able to build evolutionary family trees showing how blood cells may have developed over hundreds of millions of years.
The team also compared animal cells with single-celled organisms. This allowed them to look even further back in time and search for clues about the earliest ancestors of blood cells.
One of their most surprising discoveries involved macrophages. These are immune cells that act like the body’s cleanup crew.
They destroy harmful germs, remove dead cells, and help coordinate immune responses.
Among all human blood cells, macrophages showed the strongest similarities to single-celled organisms. This suggests that the first blood cells were probably macrophage-like cells that evolved from ancient single-celled ancestors.
The researchers also traced the history of a gene called FOS, which is active in blood cells across many different animal species.
Their analysis suggested that this gene can be traced back to a single-celled ancestor that lived around 700 million years ago.
This finding indicates that the earliest animals may have created blood cells by reusing genetic tools inherited from their single-celled predecessors. As animals became more complex, these primitive blood cells gradually diversified into the many specialized blood cells found today.
The study suggests that mast cells, which help trigger allergic reactions and inflammation, evolved from macrophages. Later, early versions of T cells and red blood cells branched off from mast cells. Meanwhile, the ancestors of B cells, which produce antibodies, appear to have evolved directly from macrophages.
By piecing together these relationships, the researchers were able to reconstruct a family tree of blood cells spanning hundreds of millions of years.
The findings reveal that the evolutionary history of life is still present inside our bodies today. The pathways that guide the development of blood and immune cells may reflect events that occurred long before dinosaurs, plants, or even complex animals existed.
The researchers believe their new method could have uses beyond studying evolution. It may also help scientists understand how diseases such as cancer develop and evolve, potentially leading to new ways to diagnose and treat these conditions in the future.
