Since Charles Darwin (1809-1882), the theory of evolution has been based on the principle of natural selection. This favors the transmission of traits best adapted to the environment. In the collective imagination, this evolution is often perceived as a linear progression toward increasing complexity. Yet nature shows that evolution has no privileged direction: it can also lead to simplified forms, loss of organs, or even true devolution.
Evolution has no direction or purpose. It explores possibilities offered by the environment and retains what works, even if it means apparent simplification. As Stephen Jay Gould (1941-2002) pointed out, complexity is merely an accidental consequence of life, not its destination. Devolution reminds us that nature does not "progress," it adapts.
N.B.:
The term devolution is not recognized as a formal concept by modern biology. It is a metaphorical description of processes of functional loss or evolutionary simplification.
In his theory "On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life" (1859), Charles Darwin never uses the word "evolution." The term appears only once, and only in the final edition (the 6th, 1872), in the closing sentence: "There is grandeur in this view of life... from so simple a beginning, endless forms most beautiful and most wonderful have been, and are being, evolved."
Darwin was wary of the word "evolution," as before him, it mainly referred to a preprogrammed development, an unfolding of an internal plan (notably in Lamarck's work), whereas his theory was based precisely on the absence of a plan or direction.
The continuous ascent toward greater intelligence, size, and perfection is a vision tinged with anthropocentrism, a misinterpretation of Charles Darwin's theory. Sometimes, the best adaptation is to regress.
Devolution is not a "regression" in the pejorative sense. It rather describes an evolutionary phenomenon where an organism loses complex traits in favor of a simpler form. It is not a return to an ancestor, but a new adaptation through subtraction. The driving force is not "regression" but selective pressure that favors simplicity when complexity becomes a burden.
Every complex structure has a cost. Each organ, cellular network, or expressed gene consumes energy, requires genetic control, and maintenance. When the environment no longer demands certain functions, the selective pressure that maintained them disappears. The species gains energy efficiency by simplifying its biological architecture.
| Organism | Lost or Simplified Trait | Adaptive Cause or Context | Comment |
|---|---|---|---|
| Cavefish Astyanax mexicanus | Loss of eyes and pigmentation | Life in total darkness, energy saving | Losing sight is not an "error" of evolution; it is a remarkable adaptation that allowed these species to conquer an extreme ecological niche. |
| Tapeworm (Taenia solium) | Disappearance of the digestive tract | Direct absorption of nutrients from the host | Extreme reduction of metabolism and loss of the digestive system, leading to complete parasitic specialization where the organism survives by directly absorbing nutrients from its host. |
| Snakes (descendants of lizards) | Forelegs Hind legs | Adaptation to burrowing Locomotion by undulation | The loss of limbs is correlated with burrowing lifestyles or undulatory locomotion, which is more efficient for moving through burrows or pursuing prey. |
| Whale (Balaenoptera musculus) | Loss of hind limbs | Complete adaptation to the aquatic environment | Hind limbs would create drag and make swimming much less efficient. |
| Penguin (Aptenodytes forsteri) | Loss of flight | Transformation of wings into flippers | Aerodynamic conversion to hydrodynamics for efficient underwater propulsion. |
| Ostrich (Struthio camelus) | Inability to fly | Adaptation to fast terrestrial running | Energy redirected to running: Residual wings are used for balance and courtship displays. |
| Leafcutter ant Atta cephalotes | Ability to digest cellulose | Symbiosis with fungus Division of labor | Having outsourced their cellulose digestion to symbiotic fungi, they have lost this physiological ability, favoring a collective specialization where each member of the colony contributes to a mutualistic food system. |
| Bird Apteryx australis (kiwi) | Reduction of wings and eyes | Nocturnal and terrestrial life in New Zealand forests | Its plumage has regressed to a downy texture similar to hair, while its terminal nostrils and hyper-developed sense of smell compensate for this simplification, making it a specialized nocturnal predator. |
| Amphibian Proteus anguinus | Loss of functional eyes | Underground life in limestone caves | Atrophied visual organs replaced by cutaneous sensitivity to light. |
How does nature, without intention or direction, produce an increasing organization of living matter, from the primitive cell to complex multicellular organisms?
The answer lies in the thermodynamics of open systems and the logic of self-organization.
Biological evolution is not progress, but an exploration of possibilities. Evolution has no purpose (no goal of moving toward complexity). Each biological transformation is simply the result of local constraints: random mutations, physical and chemical interactions, and natural selection in a given environment. Some of these constraints favor the emergence of stable structures, i.e., more organized ones.
Thus, the increasing complexity we observe in the biosphere is not a universal trend, but a collateral effect of the physics of dissipative systems.
A living system is an open system, far from thermodynamic equilibrium. It continuously exchanges matter and energy with its environment. According to Ilya Prigogine's (1917-2003) theory, these systems can self-organize when the energy flow exceeds a certain critical threshold.
N.B.:
Principle: A constant energy flow can maintain an ordered structure, as long as entropy is dissipated outward.
The transition to the eukaryotic cell is not "progress," but the result of stabilized symbiosis. A primitive cell (archaeon) integrated an aerobic bacterium, which became a mitochondrion. This process of endosymbiosis allowed for more efficient energy exploitation, thus increasing the capacity for self-organization. It is an energetic transition before being a hierarchical one.
When similar cells cooperate to better manage energy and nutrient flows, functional differentiation naturally emerges. Some cells specialize in structure, others in reproduction, and others in communication.
Each level of organization (cell → tissue → organ → organism) is not the product of a "plan" but of a progressive stabilization of interactions. The more a system exchanges energy and maintains memory (genetic, epigenetic, or chemical information), the more it can structure itself without losing its dynamic balance. From a physical point of view, maintaining an ordered and complex structure requires a constant flow of energy.
\( \text{Complexity} \approx \text{Stability} + \text{Energy Flow} + \text{Conserved Information} \)
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