When a caterpillar bites a leaf, the plant registers the damage within seconds. An electrical signal — a slow wave potential — propagates through the vascular tissue at roughly seven centimeters per minute. The signal reaches undamaged leaves on the other side of the plant, where it triggers a calcium influx, which triggers the synthesis of jasmonic acid, which activates genes that produce defensive proteins. The whole plant is warned. Leaves the caterpillar hasn't touched yet begin hardening their defenses.
This is not metaphor. The electrical signal is real — measurable with electrodes placed on the stem. The depolarization is greater than fifty millivolts. The propagation is faster than any chemical diffusion could account for. The distant leaves are genuinely responding to a signal they received from a wound they couldn't see.
The mechanism is glutamate. When cells are damaged, they release glutamate into the intercellular space. Nearby receptors — channels in the cell membrane — open in response, letting calcium in. The calcium influx depolarizes the membrane. The depolarization propagates to the next cell, which releases more calcium. The signal moves like a wave.
If this sounds familiar, it should. Glutamate is the primary excitatory neurotransmitter in the vertebrate nervous system. The receptors that detect it in plant cells — called GLRs, glutamate receptor-like proteins — are structurally homologous to the ionotropic glutamate receptors in your synapses. Same ligand-binding domains. Same transmembrane architecture. The mechanism plants use to warn their leaves about caterpillars is recognizably related to the mechanism you use to think.
The obvious interpretation is convergent evolution: two lineages independently arriving at the same solution. But phylogenetic analysis doesn't support that. The GLRs in plants and the iGluRs in animals did not evolve separately. They diverged from a common ancestor. The glutamate receptor mechanism predates the split between plants and animals — predates the existence of either kingdom. Something that was neither plant nor animal, somewhere in the Proterozoic, was already using glutamate to gate ion channels. Both lineages inherited this from the common ancestor and kept it, diverging in what they used it for.
Animals elaborated it into nervous systems. Neurons, synapses, the entire architecture of sensation and thought. Plants kept a simpler version and repurposed it for wound response — for conducting alarm signals through vascular tissue rather than neural tissue. Same molecular tool, different use. One lineage built a brain; the other built a warning network. Both innovations are more than five hundred million years old. The underlying mechanism is older than both.
What I find interesting here is the implication for how we think about nervous systems. The standard story is that nervous systems were invented — that at some point in animal evolution, neurons emerged as a novel structure, and with them came the capacity for rapid electrical signaling. This is true in the narrow sense. But the molecular substrate for electrical signaling was already present. The glutamate receptor is not a nervous-system invention. It is an inheritance, already functional, already capable of doing the basic work. The nervous system is partly an elaboration on infrastructure that was already there, that plants still use in a more minimal form.
There's a counterpart to this in technology: the internet was not invented from scratch. It assembled itself from existing phone lines, from packet-switching research done for military communications, from TCP/IP that borrowed concepts from older protocols, from hardware developed for entirely different purposes. The novel thing was the integration and the scale, not every individual component. Evolution works the same way. Novelty almost always has a substrate.
The plant's electrical signal also has a peculiar regulation mechanism worth noting. A proton pump — the AHA1 protein — actively works to shorten the signal. It restores the membrane potential, cutting off the depolarization wave. Without this pump, signals propagate longer and stronger, triggering excessive defense responses. With it, the plant calibrates. The alarm system has a built-in quieting mechanism, because an alarm that never turns off is worse than none at all.
The caterpillar bites a leaf. Glutamate floods out of the wound. Ancient receptors open. Calcium pours in. An electrical wave moves up the stem at seven centimeters per minute. Leaves that have not been touched begin to harden. The mechanism that makes this possible was not invented by the plant, or by any of its ancestors that were recognizably plants. It was inherited from something much older — something that both the plant and the caterpillar also inherited, though the caterpillar elaborated it further, turned it inward, built a nervous system around it, eventually thought about caterpillars and plants.
The signal is old. It was already there.