Insects avoid their dead instinctively, repelled by a "death stench." Now scientists have discovered that nearly all animals emit the same stench when they die, and have been for over 400 million years.
Evolutionary biologist David Rollo and a team of scientists at McMaster University in Canada stumbled upon the universal death stench while studying how cockroaches avoid their dead. After observing that the insects would avoid fluid extracted from dead cockroaches, they surmised that smell is what allowed them to recognize and avoid these dead bodies. It turned out that they were right - after an animal or insect dies, its cells release fatty acids that carry a distinct odor. This same odor acts as a repellant in ocean-going crustacians too.
And the avoid this "death stench" of fatty acids goes back over 400 million years, around the time that insects and crustacians diverged. Rollo and his colleagues believe this because insects and crustacians share the aversion, which means it most likely evolved before the two families of species split off from one another. In fact, death stench avoidance probably evolved in the ocean.
Why would this be such an ancient trait?
Recognizing and avoiding the dead could reduce the chances of catching the disease, or allow you to get away with just enough exposure to activate your immunity . . . As explained in our study, fatty acids-oleic or linoleic acids-are reliably and quickly released from the cells following death. Evolution appears to have favoured such clues because they were reliably associated with demise, and avoiding contagion and predation are rather critical to survival.
Even if the death stench was emitted from a severed limb instead of a dead body, it is still useful as a warning. Rotting limbs could be diseased - or could be a sign that predators are nearby.
So does this mean that the fear of death evolved after the lust for sex? After all, there had been millennia of humping before the proto-crustacian-insects started avoiding the death stench.