A short evolutionary tour of the clotting cascade – and a preview of a hagfish hemostasis series from TBP
When you press on a bleeding cut in clinic, you are leaning on the end result of one of the most intricate molecular systems vertebrates ever evolved. To the patient it is simple: “My blood clots.” To us, it is a tightly regulated enzyme network that appears, in recognizable form across all vertebrate groups from jawless fish to humans.
In this essay, we step back and ask a basic question: Where did this system come from? How did evolution move from simple cell clumping in early chordates to the layered cascade of serine proteases, cofactors, inhibitors, and fibrinolytic enzymes we teach in hematology today?
And then we look forward. At The Blood Project, we have now identified and characterized genes involved in the clotting process of the hagfish, the most ancient vertebrate living today. We have sequenced all of its clotting genes, and over time we will post and interpret them factor by factor. This essay is meant as the conceptual foundation for that series.
From sticky cells to fibrin clots
In mammals, we are used to teaching coagulation in terms of the extrinsic and intrinsic pathways converging on thrombin and fibrin. Evolution did not start there.
- Protochordates like sea squirts (urochordates) and amphioxus (cephalochordates) do not form fibrin clots at all. They manage hemostasis mainly by cell aggregation: circulating hemocytes clump at the site of injury.
- These organisms have many of the domains we recognize from vertebrate coagulation factors – GLA domains, kringle domains, discoidin domains, fibrinogen-related domains, and serine proteases – but no true coagulation cascade and no fibrinogen.
- Amphioxus even has a thrombin-like protease but still no bona fide fibrinogen. Whatever that protease was doing originally, it was not yet converting fibrinogen to fibrin.
The implication is striking: the vertebrate clotting system appears to have arisen in a remarkably short evolutionary window – roughly 50–100 million years between protochordates and the emergence of jawless vertebrates such as lampreys and hagfish.
Somewhere in that interval, evolution brought together:
- A thrombin-like serine protease
- A true fibrinogen molecule
- Cell-surface localization modules (like the GLA domain)
- Early cofactors and inhibitors
- Fibrinolytic factors
to create the first simplified, and integrated coagulation system. This convergence, packed into a narrow sweep of evolutionary time, set the stage for the complex, multi-layered clotting cascade that humans rely on today.
The vertebrate solution: thrombin and fibrinogen at the core
Despite all the complexity we teach, the core vertebrate innovation is simple:
Thrombin converts soluble fibrinogen into an insoluble fibrin mesh.
That reaction is unique to vertebrates. This gave early vertebrates a fast, localized, and structurally robust way to seal vascular injuries, something not achievable through cell aggregation alone. In every vertebrate group studied – jawless fish, cartilaginous fish, bony fish, amphibians, reptiles, birds, mammals – you find these core components:
- Prothrombin (PT), activated to thrombin
- Fibrinogen, cleaved to fibrin
- Factor XIII, stabilizing the fibrin clot by catalytic crosslinking
- Fibrinolytic machinery, mainly plasmin derived from plasminogen, to dissolve the clot
What changes across evolution is not the core reaction but the architecture feeding into it – how many activation steps and control points sit upstream of thrombin, and how many cofactors, feedback loops, and accessory pathways are added to modulate how much thrombin is made and when.
Four major gene families behind the cascade
When you look at the clotting system at the level of genes and protein domains, a pattern emerges. Much of its apparent complexity comes from the duplication and diversification of a few ancestral functional modules within four protein families.
- GLA–EGF–EGF–Serine protease family
- Includes FVII, FIX, FX, protein C (PC), and protein Z (PZ).
- Shares a modular structure: GLA domain (for membrane binding), two EGF-like domains (for protein–protein interactions), and a trypsin-like serine protease domain (SP).
- This family likely arose early in vertebrate evolution through serial duplications of an ancestral GLA–EGF–EGF–SP gene that diverged into distinct lineages: one giving rise to FVII/FX, another to PC/PZ (with PZ losing catalytic activity), and later FIX.
- A1–A2–B–A3–C1–C2 cofactor family
- Includes FV and FVIII cofactors.
- These cofactors are built on three internally duplicated A-domains (related to an ancient bacterial ferroxidases), followed by two discoidin (C) domains that anchor them to platelet or thrombocyte membranes.
- Their triplicated A-domain structure predates vertebrates and is also found in non-hemostatic proteins such as ceruloplasmin and hephaestin, underscoring a deep evolutionary link.
- PAN–serine protease family
- Includes FXI and prekallikrein, both carrying four “apple”/PAN domains preceding the protease domain.
- These domains likely derive from the PAN modules present in plasminogen and hepatocyte growth factor (HGF).
- The “apple”/PAN domains mediate protein assembly on charged and foreign surfaces.
- FXII family
- Includes FXII and structurally related proteins such as hepatocyte growth factor activator (HGFA).
- The FXII family is central to the contact pathway in mammals but shows striking lineage specificity: absent or nonhemostatic in early vertebrates, indicating a relatively late evolutionary role in coagulation.
The overall picture is not of a system designed or built from scratch, but of ancient pre-existing protein domains shuffled, duplicated, and repurposed to create the coagulation machinery we recognize today.
Jawless fish: the first true clotting cascades
Jawless fish (cyclostomes) – lampreys and hagfish – are the earliest living vertebrates, and they represent the first organisms with a bona fide thrombin-fibrinogen clotting system. They have:
- Hemoglobin-packed red cells
- A closed circulatory system
- And a simpler but clearly recognizable clotting system
Lampreys are currently the best-characterized jawless fish, and several key elements of the vertebrate hemostatic toolkit are unmistakably present:
- Present:
- Prothrombin
- Fibrinogen with recognizable α, β, and γ chains
- Vitamin K–dependent FVII and FX (often with multiple paralogs)
- Tissue factor (TF) activity
- Some inhibitors such as heparin cofactor II and TFPI
- Absent or not yet detected:
- FIX
- The FXI/prekallikrein family
- FXII
- A clear FV/FVIII-like cofactor remains uncertain in lamprey
Functionally, jawless fish appear to operate on a stripped-down clotting pathwat cascade:
TF + FVII → FX → thrombin → fibrinogen → fibrin
with fewer cofactors, reduced feedback loops and amplifications, and no contact activation. It is still unmistakenly a sophisticated vertebrate cascade, but far simpler than in mammals.
Hagfish, which likely diverged even earlier than lampreys based on prevailing morphological hypothesis, have been much less characterized at the molecular level. That gap is exactly where our new TBP work is focused, and why hagfish are so exciting: they are living windows into the earliest workable version of the vertebrate clotting system.
Jawed vertebrates: adding more knobs and dials
Once vertebrates evolve jaws and more complex cardiovascular systems, the clotting network expands primarily by adding new protease–cofactor modules and more processes for feedback control.
- Cartilaginous fish (sharks, rays, skates):
- Have PT, FVII, FIX, FX, PC, FV, FVIII, fibrinogen, and FXIII.
- Lack the contact pathway entirely: no FXI, prekallikrein, or FXII.
- Bony fish (teleosts, such as puffer fish, zebrafish):
- Have essentially all the vitamin K–dependent proteases and FV/FVIII cofactors.
- Often have extra paralogs (for example, multiple FVII and FIX genes), reflecting local or whole-genome duplications.
- Still lack FXI, FXII, and prekallikrein, so as in cartilaginous fish, no full mammalian-style contact pathway.
- Tetrapods (amphibians, reptiles, mammals):
- This is where the contact system emerges: FXII, FXI, prekallikrein, and high-molecular-weight kininogen.
- These added factors not only introduce another pathway to generate thrombin but also connect coagulation to inflammatory signaling through bradykinin and related peptides.
Birds are an important exception as they appear to have lost FXII, which helps explain why avian blood is relatively insensitive to contact activation, a valuable comparative teaching point when discussing “intrinsic” pathway tests.
Contact activation: a by-product of innate immunity
A key insight from comparative genomics is that the contact pathway likely began as an inflammatory system, not as a hemostatic one.
- The kinins (bradykinin, kallidin) that emerge from the contact system increase vascular permeability and pain.
- FXII, prekallikrein, and FXI share ancestry with complement-related proteases and HGF-like proteases.
- In fish, you can find kininogen but not the full contact cascade and not the “contact” domain in HMWK that drives surface activation.
The likely scenario is:
- An ancestral inflammatory/kinin system evolves for host defense.
- Its serine proteases and cofactors gradually hook into the existing clotting network.
- In tetrapods, the two systems become tightly intertwined.
By the time we reach mammals, coagulation and inflammation are deeply coupled, something we see at the bedside in sepsis, DIC, and many thrombo-inflammatory disorders.
Coagulation and complement: parallel cascades
The complement system provides another example of a modular protease cascade that expanded dramatically in early vertebrate evolution:
- Core complement components like C3 long predate vertebrates, appearing in organisms as distant as corals and horseshoe crabs.
- Ascidians have C3, factor B, MASP, and terminal complement components, but show few gene duplications within these families.
- In jawed vertebrates, duplicated complement genes (C3/C4/C5; fB/C2; MASP-1/MASP-2/C1r/C1s; C6–C9) give rise to the classical, lectin, and terminal pathways.
Coagulation shows a similar pattern, but with some important differences in timing:
- FVII/FX/PC/PT duplications appear to predate the split between jawless and jawed vertebrates.
- FV/FVIII and FIX/FX duplications, as well as many complement duplications, likely trace to the two rounds (2R) of whole-genome duplication early in vertebrate evolution.
The net effect: both systems are built from the same evolutionary toolkit – repeated gene duplication, divergence, and domain shuffling – to create cascades that can be rapidly triggered, locally contained, and tightly regulated.
Why make clotting more complex?
Compared with jawless fish, mammals operate under very different physiological constraint:
- Higher arterial pressures
- More extensively branched vascular trees and specialized microcirculations
- Higher metabolic rates and greater oxygen demands
A simple TF–VII–X–thrombin–fibrin system may be entirely adequate in a low-pressure, lower-flow circulation. But as circulatory systems became more pressurized and tissues more metabolically demanding, there is evolutionary pressure to:
- Increase sensitivity (respond to small vascular injuries)
- Increase specificity (localize the clotting response to the injury site)
- Increase controllability (turn the clotting system off quickly and avoid catastrophic thrombosis)
Adding layers of cofactors (FV, FVIII), additional parallel activation pathways (contact system), and expanding the set of inhibitors (AT, TFPI, protein C–protein S) achieves exactly that. he consequence is a highly tunable system, powerful but complex, whose intricacy we wrestle with when interpreting PT, aPTT, and the full thrombophilia panel.
Hagfish: a living test case for the “minimal” vertebrate cascade
All of the above sets up a natural question:
What does the most ancient living vertebrate actually use for clotting?
Lamprey genomics and biochemistry have given us a partial answer. But hagfish represent an equally old branch of the vertebrate tree and arguably even more primitive in some respects. Yet until recently, their clotting system at the molecular level has remained poorly defined.
At The Blood Project, we have now:
- Sequenced most of the hagfish clotting genes
- Reconstructed tan integrated coagulation cascade for this organism.
In other words, we can now move beyond inference from lamprey and puffer fish. We can examine directly:
- Which vitamin K–dependent proteases are present in hagfish
- Whether there are clear FV/FVIII-like cofactors present
- How many inhibitors and fibrinolytic factors are present
- How hagfish factors map onto the gene families and domain architectures outlined above
This will allow us to ask, with much finer resolution:
- What is the minimal set of factors needed to generate a stable fibrin clot in a vertebrate circulation?
- Which components came first, and which represent later refinements?
- How do hagfish genes illuminate the stepwise path from protochordate hemocyte clumping to the multilayered human coagulation network?
What comes next on TBP
This essay provides the conceptual scaffolding for a new TBP series that will:
- Present individual hagfish clotting genes (PT, FVII, FX, fibrinogen chains, inhibitors, and others),
- Compare them to their lamprey, fish, and mammalian orthologs
- Use those comparisons to reconstruct the early steps in vertebrate clotting evolution in a clinically meaningful way.
Over time, we will post:
- Gene-by-gene summaries with domain diagrams
- Comparative sequence alignments and key residue maps
- Short interpretive notes highlighting what each factor tells us about modern hemostasis and human disease (for example, why certain human factor deficiencies clinically resemble “earlier” or “later” in evolutionary states)
Hagfish may look alien, but their clotting system is family. Understanding how they stop bleeding will deepen our understanding of how our own patients do the same, and why the cascade sometimes fails in ways that cause bleeding, thrombosis, or both.
Stay tuned on The Blood Project for the hagfish series, built on the evolutionary story you have just read.