The vital fluid circulating within your vascular system likely occupies little of your daily contemplation.
Much like the incessant neural activity within your cranium or the rhythmic ingress and egress of your respiration, it operates unobtrusively in the background—an often-unnoticed element of your physiology crucial for sustaining life.
However, to a segment of the scientific community, this crimson ichor, propelled through every corporal extremity with each cardiac pulsation and responsible for the conveyance of indispensable oxygen to your cellular constituents, represents a profound enigma.
The vast majority of life forms inhabiting this remarkable planet are endowed with blood… but the precise genesis of this essential biological liquid remains a compelling question.
In a monumental undertaking, an international consortium, spearheaded by researchers at Kyoto University in Japan, has meticulously charted the phylogenetic trajectory of blood cells, extending back an astonishing 700 million years. Their discoveries indicate that these cells were not spontaneously generated subsequent to the proliferation of multicellular organisms.
Instead, it appears that blood evolved through the repurposing and refinement of genetic mechanisms inherited from ancestral unicellular organisms that predated the emergence of animal life by hundreds of millions of years.
“These revelations, representing the apex of our investigative endeavors, deeply resonate with me, as they illuminate the fact that the differentiation pathways of vertebrate blood cells encapsulate the 700-million-year evolutionary chronicle of these cellular entities,” stated immunologist Hiroshi Kawamoto, a principal investigator at Kyoto University, who spearheaded this groundbreaking study.
When contrasted with certain other facets of evolutionary history, elucidating the lineage of blood is an immensely challenging pursuit. Skeletal structures, dermal coverings, avian plumage, and exoskeletons are preserved as fossils. Cellular structures, conversely, typically undergo rapid decomposition.
Consequently, deciphering their origins necessitates an inferential methodology.
The fundamental unit employed by the researchers to reconstruct this evolutionary narrative is the transcriptome—essentially, a molecular snapshot that delineates the spectrum of gene expression, revealing which genes are actively engaged and which remain quiescent within any given cell type.
Transcriptome data were meticulously gathered from an extensive array of genera, encompassing humans, rodents, zebrafish, ascidians (tunicates), echinoderms, arthropods, annelids, poriferans, and several unicellular species.
Subsequently, the investigators sought to identify convergent patterns, positing that if two disparate cellular lineages exhibit remarkably similar regulatory apparatus, they likely share a common ancestral cellular blueprint.
While it is conceivable for distinct organisms to independently develop analogous traits, the probability of such convergent evolution diminishes significantly with the depth, ubiquity, and complexity of the observed homologies.
The analytical outcomes indicated that the earliest iteration of blood cells bore little resemblance to the highly specialized, optimized oxygen carriers currently functioning within the human organism.
Rather, they were likely rudimentary, amoeboid entities akin to macrophages, the significant leukocytes that constitute the vanguard of the immune system—mobile phagocytes adept at internalizing and degrading invasive microorganisms.
Yet, their existence was not entirely novel.
Upon comparing the transcriptional profiles of blood cells with those of contemporary unicellular organisms closely allied with the animal models in their study, the researchers detected notable congruences.
Certain unicellular species exhibited genetic blueprints remarkably proximate to those of macrophages, including the process of phagocytosis—the mechanism by which cells internalize and metabolize particulate matter.
To further probe this association, the researchers focused on a gene, denoted as Fos, which was consistently identified in both animal blood cells and unicellular organisms.
This gene plays a pivotal role in regulating cellular proliferation and morphogenesis, and its recurrent presence across such distantly related taxa rendered it a primary candidate for investigation.
They selected a unicellular organism and experimentally amplified Fos gene expression. Instead of exhibiting the typical cohesive behavior, the cells maintained an isolated, amoeba-like morphology.
These experimental outcomes suggest that the genetic framework responsible for macrophage-like functionality originated in unicellular organisms hundreds of millions of years prior to the divergence of animals and unicellular eukaryotes from a shared progenitor.
From this evolutionary juncture, the researchers posit that blood cells bifurcated into two principal phylogenetic branches.
Emerging from the macrophage-like ancestral blood cells, a secondary major lineage eventually diverged: the precursors of mast cells, which serve as critical alarm signals in the immune response to external threats.
These mast cells subsequently gave rise to T lymphocytes, erythrocytes, and platelets, while macrophages continued to differentiate into B lymphocytes responsible for antibody production.
According to the researchers, these findings hold significant promise for unraveling the evolutionary origins of pathologies such as cancer, and concurrently, they underscore a profound ancestral legacy tracing back to our most rudimentary beginnings.
“Upon internalizing the realization that this deep-seated legacy from eons past is presently circulating within my body as blood cells, I experience a heightened sense of connection to our ancient forebears,” expressed immunologist and lead author Yosuke Nagahata of the Institute of Evolutionary Biology in Spain.
The results of this investigation have been formally published in the Proceedings of the National Academy of Sciences.
