Why the same principles that explain the elephant's trunk and the butterfly's wing might also explain money, monarchy, and your morning ritual.
What do a gene, a gut bacterium, a religious ritual, and a national currency have in common? At first glance, not much. But a revolutionary extension of evolutionary theory suggests that these seemingly disparate things all belong to a special class of entities known as "historical kinds."
For decades, evolutionary biology has focused on explaining the adaptation of organisms to their environments. Now, a groundbreaking framework is expanding this view, proposing that the same core principles can explain the origin and evolution of a vast array of stable entities in both biology and culture. This isn't just about the evolution of physical traits; it's about understanding how everything from cell types to social institutions emerges, persists, and changes through history. This journey will take us from the logic of natural selection to the mechanisms that give rise to our complex social world, revealing a unified theory of history's great diversifiers.
Explains adaptation of organisms through natural selection
Explains emergence and persistence of cultural institutions
To understand the leap forward, we must first grasp the concept of historical kinds. Traditional evolutionary theory excels at explaining "eternal kinds"—things defined by unchanging, intrinsic properties, like the chemical elements. Historical kinds, in contrast, are defined by their lineage and their history5 .
Members of a historical kind share a common ancestry or a line of descent. A species is a classic biological example; its members are alike because they are historically connected, not because they all match a perfect "essence"4 5 .
These kinds, once formed, can evolve and change through time in a way that is somewhat independent of their surroundings. They become stable "clumpings" in the fabric of history4 .
This concept was powerfully articulated by philosophers of science who noticed that many of the kinds studied in biology and the social sciences don't obey eternal laws but are instead held together by historical connections5 .
How does a historical kind maintain its identity over time, even as it changes? The answer lies in the theory of Homeostatic Property Clusters (HPC)4 .
Think of a historical kind not as a single, essential thing, but as a cluster of properties that are reliably found together. What keeps this cluster stable is a network of underlying mechanisms that maintain these correlations. This "homeostasis" is not rigid but is a stable, self-maintaining state.
A "cell type," like a neuron, is defined by a cluster of properties (shape, function, gene expression). This cluster is maintained by a network of genetic and regulatory mechanisms that ensure new neurons are produced with these same properties4 .
"Monarchy" is defined by a cluster of functions—religious leadership, political mediation, law-making, military command. These functions interact and reinforce one another, creating a stable institutional role that can be reproduced across generations and cultures5 .
This framework allows us to see that the emergence of a new historical kind—whether a new organ in an animal or a new institution in a society—is the emergence of a new, self-maintaining network of components.
| Feature | Eternal Kinds (e.g., Gold) | Historical Kinds (e.g., Monarchies) |
|---|---|---|
| Defining Principle | Intrinsic, unchanging essence | Historical lineage and descent |
| Identity Over Time | Static and uniform | Stable but capable of change and variation |
| Primary Bond | Shared physical properties | Shared history and causal connections |
| Inductive Power | Based on universal laws | Based on lineage-specific patterns |
| Examples | Chemical elements, fundamental particles | Biological species, cultural institutions, cell types |
Table 1: Key Characteristics of Historical Kinds
The theory of historical kinds finds a strong foundation in the extended evolutionary synthesis, which moves beyond a narrow focus on genes and adaptation. It recognizes that evolution involves multiple processes that can give rise to new and stable lineages4 .
The evolution of complex life is a story of the progressive emergence of new historical kinds.
The primordial historical kind. Genes are historical entities that are replicated and modified over deep time.
A major evolutionary transition. A new cell type originates when a network of genes stabilizes into a new, self-perpetuating state4 .
The origination of a new cell type is not just an adaptation; it is the creation of a new building block for evolution to work with. It represents a new quasi-independent lineage within the developing organism.
Perhaps the most thrilling application of this theory is to culture. If biological evolution can produce historical kinds like genes and cell types, could cultural evolution produce its own kinds? The evidence suggests a resounding yes.
The idea of cultural evolution is almost as old as Darwinism itself. Shortly after The Origin of Species was published, figures like Spencer and Tylor began applying evolutionary thinking to human societies and institutions7 . They viewed human cultures and their structures—like myths, laws, and kinship systems—as entities that descend from previous forms with modification7 .
Culture, in this view, is "that complex whole which includes knowledge, belief, art, morals, law, custom, and any other capabilities and habits acquired by man as a member of society"7 . The key word is "acquired"—not inherited through genes, but passed on through learning and imitation.
The stirrup is a powerful example of a cultural historical kind. This seemingly simple tool is part of a reproductive lineage:
The first stirrups appeared in Asia.
The design was copied and spread westward.
New stirrups closely resembled past models in form and function.
This process of reproduction continued across generations and cultures5 .
But the stirrup's story doesn't end there. Its adoption by Charles Martel's Frankish army in the 8th century enhanced the effectiveness of cavalry. This military innovation had a cascading effect on the social structure, helping to cement a feudal system where armored knights on horseback became the center of military and political power5 . A change in a material artifact led to the transformation of a social order, demonstrating how cultural kinds can have profound and unexpected evolutionary consequences.
Why do institutions like monarchy or practices like marriage appear in historically unrelated societies? According to the theory, they are multi-functional kinds5 .
Monarchy, for instance, is not defined by one job. It typically clusters several functions—religious, political, judicial, and military. These functions interact; success in war strengthens political power, which reinforces religious authority. This interlocking network of functions makes the institution stable and likely to re-emerge in different societies facing similar organizational challenges, even without a shared cultural ancestry5 .
| Domain | Historical Kind | Key Properties in the Cluster | Maintaining Mechanisms |
|---|---|---|---|
| Biology | Biological Species | Shared morphology, genetic compatibility, ecological role | Gene flow, shared selective pressures, developmental constraints |
| Biology | Neuron Cell Type | Specific shape, electrochemical signaling, synaptic function | Gene regulatory networks, cellular signaling pathways |
| Culture | Monetary Currency | Store of value, medium of exchange, unit of account | Legal tender laws, central banking, public trust |
| Culture | Monarchy | Hereditary succession, religious authority, political leadership | Ritual, law, interlocking social and political functions |
Table 2: Examples of Historical Kinds in Biology and Culture
To see evolution in action, scientists have turned to experimental evolution, using laboratory experiments to observe evolutionary dynamics in real-time3 . These studies provide some of the most direct evidence for how new historical kinds can originate.
In 1988, biologist Richard Lenski started a simple experiment that would become a cornerstone of evolutionary biology. He founded twelve populations of the bacterium E. coli from a single ancestor and began growing them in a controlled laboratory environment. Every day, a sample of each population is transferred to fresh growth medium, allowing for continuous evolution. This experiment has now run for over 60,000 generations, providing an unparalleled window into the evolutionary process3 .
For the first 30,000 generations, the twelve populations evolved in parallel, becoming better adapted to the lab environment. Then, in one population, something unprecedented happened. The bacteria in this population suddenly evolved the ability to consume citrate, a chemical present in the growth medium that E. coli normally cannot use in the presence of oxygen3 .
This was not a minor adjustment; it was the origin of a new metabolic capability—a potential new "historical kind" of E. coli. By comparing the modern citrate-eating (Cit+) bacteria to the frozen fossils from earlier generations, researchers could unravel the precise sequence of mutations that led to this innovation. It wasn't a single mutation but a series of steps that, over time, paved the way for this new function to emerge3 .
| Generation (Approx.) | Key Evolutionary Event |
|---|---|
| 0 | Experiment begins with 12 identical populations. |
| 2,000 | All populations show significantly improved fitness in the lab environment. |
| 10,000 | Populations continue to adapt, but at a slower rate. Mutations accumulate. |
| 31,500 | One population (Ara-3) evolves the ability to aerobically metabolize citrate (Cit+). |
| 33,000 | The Cit+ lineage completely dominates its population. |
| 50,000+ | The experiment continues, with the Cit+ lineage continuing to evolve and refine its new capability. |
Table 3: Simplified Timeline of Major Events in the LTEE
Researchers exploring the origins of historical kinds, both in the lab and in the field, rely on a set of powerful conceptual and technical tools.
| Tool or Concept | Function in Research |
|---|---|
| Evolve and Resequence (E&R) | A method where organisms are evolved in the lab and their genomes are sequenced before and after to identify the precise genetic changes behind adaptation3 . |
| Homeostatic Property Cluster (HPC) | A theoretical framework that guides scientists to look for the network of mechanisms that maintain a kind's stable identity over time4 . |
| Frozen Fossil Record | Preserving samples (like bacterial populations) at different time points, allowing direct comparison between ancestors and descendants3 . |
| Experimental Evolution | Using controlled lab or field experiments to observe evolutionary dynamics directly, testing hypotheses about how new kinds originate3 . |
| Multi-Functional Analysis | In cultural evolution, this involves dissecting an institution or practice into its core, interlocking functions to understand its stability and recurrence5 . |
Table 4: Research Toolkit for Investigating Historical Kinds
Controlled experiments allow direct observation of evolutionary processes.
Advanced sequencing and computational methods reveal evolutionary patterns.
Understanding how interconnected components maintain stability in historical kinds.
The theory of historical kinds offers a profound extension of evolutionary thinking. It shows that the same fundamental logic—the emergence of stable, self-perpetuating lineages through historical processes—can help us understand the great diversifiers of life and culture.
From the first cell that decided to become a neuron to the first community that institutionalized leadership as monarchy, the universe has repeatedly discovered how to create new kinds of things that persist and evolve.
This unified view does not reduce culture to biology. Instead, it suggests that both domains are governed by a deeper set of principles about how complexity and novelty arise in history. By identifying the mechanisms that create and maintain historical kinds, we gain a powerful new lens for exploring everything from the origins of life's complexity to the deep forces that have shaped, and continue to shape, the human world. The story of evolution is no longer just the story of the fitter, but the story of the new, the stable, and the historically contingent.