For over 700 million years, the blood coursing through animal bodies—humans included—has carried a profound evolutionary narrative etched into its cellular makeup. Recent research spearheaded by a team at Kyoto University has unveiled a remarkable chronicle of blood cell evolution, tracing their origins back to single-celled ancestors from the dawn of multicellularity. This new study not only deepens our comprehension of hematological and immunological development but also reconstructs the evolutionary journey that forged the blood cells essential to animal survival.
All animals possess blood cells, yet the diversity of these cells’ lineages and functions reflects the complex evolutionary pressures each species has encountered. Despite advances in molecular biology, the evolutionary underpinnings of blood cells—how such diverse cell types emerged and specialized—remained shrouded in mystery. Seeking to illuminate this dark corner of evolutionary biology, Kyoto University scientists devised an innovative method to systematically analyze gene expression patterns across a spectrum of blood cell lineages from multiple animal species.
Central to their approach was the comparative analysis of transcriptomic data to create phylogenetic trees that articulate the evolutionary relationships among blood cell types. Unconventionally, the researchers incorporated unicellular organisms into this framework to pinpoint genetic commonalities bridging single-celled life and the blood cells of multicellular animals. This bold inclusion allowed them to situate blood cell origins firmly within the broader evolutionary narrative that spans from unicellularity to complex organisms.
Their analyses revealed that among contemporary blood cells, macrophages exhibit a gene expression profile most closely aligned with unicellular organisms. Macrophages, known as professional phagocytes, are immune cells that engulf and digest cellular debris and pathogens. This remarkable resemblance suggests that macrophage-like cells formed the primordial blood cell type at the inception of multicellular life, roughly coinciding with the advent of animals around 700 million years ago.
A pivotal gene called FOS, consistently expressed across diverse blood cell types and species, acts as a molecular marker of this ancient legacy. Tracing FOS back to single-celled ancestors indicated that these early organisms already harbored genetic blueprints later repurposed to develop blood cells. This evolutionary recycling of genes underscores a vital mechanism by which complexity arises: through modification and specialization of existing molecular tools.
Following macrophages’ emergence, the evolutionary tree branched out. Mast cells, critical in inflammatory and allergic responses, diverged from the macrophage lineage. The more specialized T cells and erythrocytes—responsible for adaptive immunity and oxygen transport, respectively—then evolved from mast cell ancestors. Meanwhile, B cells branched off independently from macrophages after the initial mast cell segregation, thereby shaping the diverse repertoire of vertebrate immune components observed today.
This phylogenetic reconnection challenges conventional perspectives that position blood cell types as discrete entities. Instead, it presents blood cells as evolutionary mosaics, integrating ancient genetic traits with newly evolved functionalities. The stepwise diversification elucidated by the Kyoto team paints a dynamic picture, where immune and circulatory cells are interwoven through deep evolutionary time, reflecting adaptative pressures to combat infections and maintain homeostasis.
By reconstructing a detailed family tree of blood cells extending over hundreds of millions of years, the research highlights how differentiation pathways within present-day vertebrate hematopoietic systems mirror the profound legacy of unicellular forebears. Such an evolutionary lens not only broadens our understanding of blood cell biology but also qualitatively informs immunology and disease pathogenesis, hinting at ancestral genetic mechanisms that may influence susceptibility and resistance to modern pathogens and malignancies.
This milestone was made possible by breakthroughs in transcriptome profiling and computational phylogenetics, enabling comparisons across species previously obscured by the absence of comprehensive genomic datasets. The methodology developed promises new avenues to dissect the evolutionary origins of pathologies like cancer, whose cellular bases are closely tied to developmental and evolutionary programs embedded within blood and immune cells.
As team leader Hiroshi Kawamoto reflects, the profound realization that circulating blood cells embody a living record of evolutionary history spanning 700 million years is intellectually and emotionally stirring. This concept bridges the gap between ancient life forms and modern humans, grounding our biological existence in a continuum of cellular innovation and adaptation.
First author Yosuke Nagahata shares a personal resonance with these findings, remarking on the intimate connection to distant ancestors evoked by considering blood cells as legacy bearers of unicellular organisms. By decoding the genetic threads threading through time, this research strengthens the bridge between evolutionary theory and contemporary biomedical science.
Ultimately, the Kyoto University team’s study not only advances evolutionary biology but also lays a foundational framework for biomedical research, with the potential to unravel complex diseases and foster innovative therapeutic development. As we peer into the microscopic chronicles inscribed within our blood, we gain both a scientific and philosophical appreciation of life’s enduring legacy—a bloodline forged in the crucible of deep evolutionary time.
Subject of Research: Not applicable
Article Title: Animals have expanded the evolutionary legacy of unicellular ancestors in blood cells
News Publication Date: 29-May-2026
Web References: http://dx.doi.org/10.1073/pnas.2528110123
References: Kyoto University research publication in Proceedings of the National Academy of Sciences, 2026
Image Credits: KyotoU / Yosuke Nagahata
Keywords: Blood cells, Macrophages, Mast cells, T lymphocytes, B lymphocytes, Evolutionary biology, Hematology, Immune cells
