March 2015, updated July 2019

Where's the evolution?

Parasites and pathogens don’t arise de novo from the primordial slime. They evolve from other organisms (usually from other parasites and pathogens). To understand how this happens, it helps to remember that to these bugs, the human species is a habitable ecosystem. Just as an uninhabited island can be invaded by a new species from a neighboring island, a living body can be colonized by a new bug from a nearby victim. If conditions are right and if the invader can survive and reproduce in the new host, the invader may evolve adaptations to the new host ecosystem resulting in a specialized relationship with that host over many generations. Eventually, the parasite lineage that invaded the new host may evolve so many differences from the source population, that it constitutes a separate species. In bedbugs, we are seeing this process in action.

Genetic and anatomical evidence suggest that bedbugs (Cimex lectularius) came to humans from an unexpected animal. When our early human ancestors evolved in Africa, some of them lived in caves, along with another notable cave-dweller: bats. Those bats colonies were parasitized by bloodsucking bugs. Some of the insects began to feed on their human neighbors as well. Around 250,000 years ago, the bat and human bedbug lineages began to diverge from one another. When humans left the caves and began living in other shelters, they took their parasites with them.

New research shows that the bat and human bedbug lineages — though both currently classified as the same species and capable of interbreeding — are in the process of becoming distinct species. The two lineages are broadly similar at a genetic level, but notable differences are accumulating. Of course, as we would expect since human bedbugs are frequently exposed to insecticides and bat bedbugs are not, gene versions that code for pesticide resistance are common in human bedbugs (where such gene versions would have been favored) and absent in bat bedbugs. But researchers also found a variety of differences throughout the genome. The two lineages have physical differences as well. For example, human bedbugs tend to have longer legs, and bat bedbugs have shorter, sturdier ones. Researchers have hypothesized that long legs may be an adaptation for escaping from humans or for moving between human dwellings.

It’s not surprising that that these two lineages have evolved so many distinguishing features. There is little evidence of gene flow (i.e., interbreeding) between the two lineages, and their hosts have quite different bodies and lifestyles. Humans are nearly hairless and sleep in beds at night. Bats are hairy and sleep in roosts during the day. We would expect that the traits favored by natural selection for feeding on the two hosts would be different.

Bedbugs have clearly invaded the human “island” and are here to stay, while other species have tried to make the leap and failed. Diseases like Ebola and rabies are able to invade the human “ecosystem” from their original hosts but don’t evolve enough adaptations to their new environment to set up shop permanently. For example, Ebola normally lives in bats. Every so often, the Ebola virus invades a human host (either from bats or from another wild animal infected by a bat). The virus circulates among humans for a little while, causing an outbreak, but then then eventually dies out. Such pathogens are called zoonotic infections.

In other cases, the association between a parasite or pathogen and its host is so tight that their evolutionary paths become intertwined. Coming back to the human island analogy, imagine a large continent on which a widely distributed species lives. What would happen to that species if plate tectonics caused part of the continent to break off and move away from the rest of the landmass, forming an island? Unless the species were able to disperse over water, eventually (over many millennia) the two halves of the population would diverge and become separate species. This process is known as vicariance, and it also occurs in parasites and pathogens whose host lineage speciates, where it is known as cospeciation. For example, six million years ago, the common ancestor of humans and chimpanzees was probably sitting around in Africa scratching at the lice that lived on its hairy body. When that single ape lineage split into two, each took a group of lice with them. The early chimp ancestors had one set of lice, and the early human ancestors had another. Once those apes stopped breeding with one another, their lice had to as well, since there were no longer opportunities for the two groups of lice to mate with one another. Over many generations, as their hosts diverged, so did the lice, eventually forming the two species we have today: Pediculus humanus lives on the human head, and P. schaeffi lives on chimpanzees.

When vicariance between parasites and their hosts occurs, we can tell because their family trees line up, speciation event for speciation event. The phylogeny below shows the relationships among primates (as well as one rodent) and their lice. You can see that, for the most part, their phylogenies mirror one another: when old world monkeys split from other primates, their lice split from other lice, etc. This indicates vicariance. However, you’ll also see one place where the phylogenies don’t align. Because we have two distinct parts of the body with hair, humans have two lice species, one that lives on the head and one that lives in the pubic hair (Pthirus pubis). Our pubic lice came to us, not through vicariance, but through dispersal (as bedbugs did). Some lice from a gorilla ancestor found themselves on a human ancestor and managed to stick it out and make a living on a new host.

The evolutionary history of human parasites and pathogens is variable, but is full of familiar patterns. Some have been with us since we first became human (e.g., our head lice), some are new arrivals (e.g., our pubic lice), and some are newer still and have just begun to diverge from their closest relatives (e.g., our bedbugs). The evolutionary processes that account for these divergences and adaptations operate at many levels in the natural world — whether one is contemplating the grand sweep of evolutionary history as continents break apart, battling an Ebola outbreak in West Africa, or just nervously scratching at a red welt and eyeing one’s mattress with suspicion.