A reptilian is any member of the class Reptilia: air-breathing vertebrates with scaly skin, internal fertilization, and eggs that can develop on land. The group includes roughly 10,000 living species spread across four major orders: lizards and snakes (Squamata), turtles (Testudines), crocodilians (Crocodylia), and the tuatara (Rhynchocephalia), a single lizard-like species found only in New Zealand. The word “reptilian” also shows up in popular psychology, where it refers to a supposed primitive layer of the human brain, though that idea has largely been discredited by modern neuroscience.
What Makes a Reptile a Reptile
Reptiles share a handful of traits that set them apart from amphibians, mammals, and birds. Their skin is covered in scales made of keratin, the same protein in human fingernails. But the real waterproofing comes not from the scales themselves but from a layer of lipids embedded in the outer skin. When researchers extracted those lipids from shed snake skin, water loss increased up to 15-fold. Removing or damaging the keratin alone only doubled water loss. That lipid barrier is what allowed reptiles to colonize deserts, grasslands, and other dry habitats that amphibians never could.
Reptiles reproduce through internal fertilization, and most lay eggs with a tough, protective shell and internal membranes (the amniotic egg). This was a pivotal adaptation. Amphibians need water for their jelly-coated eggs, but the amniotic egg carries its own fluid environment, freeing reptiles from ponds and streams entirely. Not all reptiles lay eggs, though. A significant number of species, particularly those living in cold climates, give live birth. The common lizard, for example, lays eggs in mountain populations but carries live young in lowland plains. Cold temperatures appear to be one of the strongest pressures driving species toward live birth, since keeping eggs inside the body lets the mother regulate their temperature more effectively.
Cold Blood and Energy Efficiency
Reptiles are ectotherms, meaning they rely on external heat sources to regulate body temperature. You’ll see lizards basking on rocks or turtles floating at the water’s surface for exactly this reason. This is sometimes called “cold-blooded,” but it’s a misleading label. A desert lizard in direct sun can have a body temperature matching or exceeding that of a mammal.
The real difference is metabolic. When scientists compared a lizard and a mouse of the same weight at the same body temperature, the mouse burned energy at roughly eight times the rate. Its internal organs were proportionally larger, packed with more mitochondria (the cell’s energy generators), and those mitochondria had greater membrane surface area for producing fuel. This means reptiles need far less food to survive. A snake can go weeks or months between meals, while a mouse-sized mammal would starve in days. That efficiency is a genuine advantage in environments where food is scarce or unpredictable.
Hearts, Lungs, and Circulation
Most reptiles have a three-chambered heart: two upper chambers (atria) and one lower chamber (ventricle). This means oxygen-rich blood from the lungs and oxygen-poor blood returning from the body partially mix in that shared ventricle. It sounds inefficient compared to a mammal’s four-chambered heart, but reptiles’ lower metabolic demands make it workable. Crocodilians are the exception. They have a full wall dividing the ventricle, giving them a true four-chambered heart similar in structure to what you’d find in birds and mammals.
Senses Beyond Sight and Smell
Reptiles perceive the world through some sensory channels that humans lack entirely. Snakes and many lizards use a structure called the Jacobson’s organ (or vomeronasal organ), located in the roof of the mouth. When a snake flicks its tongue, it’s collecting chemical particles from the air and pressing them against this organ. The signals travel through a dedicated neural pathway to the brain’s hypothalamus, influencing reproductive behavior, defensive responses, feeding decisions, and hormone release. It functions like a second, more specialized nose tuned to detect pheromones and other non-volatile chemical cues that regular smell would miss.
Some pit vipers and pythons also have heat-sensing pits along their jaws that detect infrared radiation, letting them locate warm-blooded prey in total darkness. Combined with the vomeronasal system, these senses give many reptiles a rich chemical and thermal picture of their environment that compensates for eyesight that is often less acute than a bird’s or mammal’s.
How Long Reptiles Have Been Around
Reptiles are among the oldest land vertebrates. Molecular studies have long placed the origin of the amniote group (the lineage that includes reptiles, birds, and mammals) at around 319 million years ago, during the mid-Carboniferous period. But a 2025 study in Nature pushed that timeline back significantly. Fossilized footprints from a clawed, amniote-like animal found in southeastern Australia date to the early Tournaisian stage, roughly 358 million years ago, near the boundary between the Devonian and Carboniferous periods. That pushes the likely origin of amniotes back by at least 35 to 40 million years beyond previous estimates.
The amniotic egg was the key innovation. By enclosing the embryo in fluid-filled membranes, it broke the reproductive tie to water and opened up vast stretches of dry land. From those early amniotes, reptiles diversified into the dinosaurs, pterosaurs, marine reptiles, and eventually the birds that are technically their living descendants.
The “Reptilian Brain” in Humans
You may have encountered the term “reptilian brain” in self-help books, psychology articles, or stress management guides. It comes from the triune brain theory proposed by neuroscientist Paul MacLean in the 1960s. MacLean argued that the human brain evolved in three layers: a “reptilian complex” at the base handling instincts and survival, a “limbic system” in the middle handling emotions, and a neocortex on top handling rational thought. The idea was that your fight-or-flight reactions come from this ancient reptilian layer, essentially a lizard brain buried inside your skull.
It’s a compelling metaphor, but modern neuroscience considers it inaccurate. The brain did not evolve by stacking new layers on top of old ones. Basic neural regions are shared across all vertebrates and differ mainly in proportion and extent, not in age or independence. Brain structures don’t operate in isolation the way MacLean described. Emotion and cognition are not separate systems housed in separate regions; they’re deeply intertwined processes running across interconnected networks. The limbic system, supposedly the “emotional brain,” is involved in memory, learning, and spatial navigation as well. And the brain doesn’t just react to stimuli. It constantly predicts internal and external needs and adapts in advance.
A 2022 review in Frontiers in Psychiatry called the triune model “an outdated, inaccurate model” and warned that building research hypotheses on it can lead to faulty assumptions. So while “reptilian brain” remains a popular shorthand for primal instincts, it doesn’t reflect how brains actually work, in humans or in reptiles.