November 2010

If you are trying to stay healthy this cold and flu season, you may find yourself washing your hands frequently and avoiding crowded places like schools and airports. That’s because most infectious diseases that we are familiar with are passed from human to human — and the more human germs you come into contact with, the more likely you are to have one make its home in your body. However, on evolutionary timescales, pathogens don’t necessarily respect species boundaries. Biologists have discovered more and more cases in which diseases have passed from another species to humans. And, as you might expect, the more closely related the other species are to us, the easier it seems to be for the pathogen to make this jump. The most recently discovered case of disease swapping among species involves the deadliest strain of malaria, Plasmodium falciparum, which causes more than a million deaths each year.

Where's the evolution?

Up until two years ago, biologists thought that this parasite had been plaguing us since the dawn of human history. P. falciparum‘s closest known relative was a species of malaria that infects chimpanzees, so it only made sense to hypothesize that the common ancestor of these parasitic species infected the common ancestor of humans and chimps. When our own lineage split from that of the chimps around six million years ago, each took a population of malaria parasites with them and these parasites evolved into separate species along with their hosts … or at least that’s what scientists thought, up until genetic testing uncovered previously unknown strains of malaria infecting other great apes.

This fall, an international group of researchers announced that they’d discovered evidence that supports a very different hypothesis about the evolutionary origins of human malaria. The researchers were studying great apes infected with HIV’s close relative, SIV (simian immunodeficiency virus). They were interested in whether parasites like malaria are particularly bad for SIV-positive animals, as is the case with HIV infections in humans. The SIV study called for screening hundreds of wild chimps and gorillas — a daunting task since these animals are protected and human contact is regulated and limited. Luckily — or perhaps unluckily for the researchers in charge of collecting samples — you don’t need a syringe to learn about the disease strains infecting a gorilla; a pooper scooper will do just fine. Feces picked up off the forest floor contain virus particles, parasites, and DNA from the host animal. The researchers collected hundreds of fecal samples from wild animals and extracted genetic material from them to learn more about the animals and their infections. When they compared malaria strains infecting different primates, the results were a surprise.

The researchers used the genetic sequences from the malaria strains to reconstruct the evolutionary relationships among the strains. As you can see from the phylogeny below, the human malaria strains form a tiny twig on a big branch of gorilla malaria parasites. They are much more closely related to gorilla strains than to chimpanzee strains. This means that P. falciparum probably made the jump to human hosts from gorillas sometime after our lineage split from that of chimpanzees.

From this tree, a few additional things are clear. First, because the chimp strains and gorilla strains don’t all group together and are scattered on different branches of the tree, it seems that malaria parasites have switched hosts (e.g., jumped from chimps to gorillas) frequently. Second, because all the sequences from human malaria are grouped together on a single branch, it seems that the human strain of malaria made the jump to humans only once.

In retrospect, it’s not surprising that malaria has jumped around from chimps to gorillas to humans so frequently. After all, HIV has jumped to humans at least five times from chimps, gorillas, and sooty mangabeys, and some human lice also trace their ancestry to gorilla lice. Humans, chimps, and gorillas are close evolutionary relatives, and have inherited similar physiologies from our recent common ancestor. So from a parasite’s perspective, making one’s home in a human may be similar to making a home in gorilla or chimp. This means that switching hosts from one great ape to another may not be that unlikely.

While new infectious diseases acquired from other organisms are certainly a concern, chimpanzees and gorillas probably have more to fear from our diseases than we do from theirs. Many human pathogens are capable of infecting other great apes, whose precarious population sizes add further concern. To guard against our close evolutionary relatives picking up human diseases, the researchers who did fieldwork in Africa for this study stayed at least 35 feet away from the apes, were vaccinated against diseases that might be passed to the wild animals, and even carried their own waste out of the forest to avoid exposing the apes to human parasites that might make a leap in the other direction. Unfortunately, the researchers also found that while such safeguards are effective, they are no guarantee against wild animals being infected with human pathogens. In the course of their research, the scientists discovered that human malaria strains have infected captive bonobos. These malaria parasites contained genes for drug resistance, a sure sign that the malaria infection was passed from human populations where malaria is treated with particular drugs. The acquisition of new infectious diseases from close evolutionary relatives is, apparently, a two way street.