Swine flu has made the world all too aware of the possibility of diseases making the leap from animal hosts to human ones. Now, we know that another disease made a similar transition from chimpanzees to humans, several thousand years ago. This particular infection is caused by a parasite, and a very familiar and dangerous one – Plasmodium falciparum, the agent responsible for malaria.
Transmitted by the bite of mosquitoes, P.falciparum infects over 500 million people every year. Its closest relative is a related parasite, Plasmodium reichenowi, which infects chimpanzees. Leading an international research team, Stephen Rich from the University of Massachussetts has discovered that P.reichenowi is no mere relative – it’s actually P.falciparum‘s ancestor.
Rich compared the genes of the two species to build a Plasmodium family tree, which showed that all of the 133 known strains of P.falciparum, from all parts of the world, are united one a single branch on the P.reichenowi lineage. The stem of that branch represents a single event where P.reichenowi crossed the species barrier from chimps to humans.
The new study was possible because of eight newly collected samples of P.reichenowi from wild and captive chimps. Until now, only a single sample of this species had ever been isolated. Armed with fresh samples, the team focused their attention on three genes – cytB, clpC and 18s rRNA. They found that those of P.reichenowi are very varied, much more so than its genetically uniform cousin P.falciparum (even though we have over 16 times as many samples of the latter). Chances are that any two samples of P.reichenowi are more genetically distinct that either one is to P.falcarium.
While the rapid expansion of human cities has been detrimental for most animals, some have found ways of exploiting these brave new worlds and learned to live with their prolific inhabitants. The Northern mockingbird is one such species. It’s very common in cities all over America’s east coast, where it frequently spends time around humans. But Douglas Levey from the University of Florida has found that its interactions with us are more complex than anyone would have guessed.
The mockingbird has the remarkable ability to tell the difference between individual humans, regardless of the clothes they wear. After less than a minute, they can tell one person from another and adjust their responses according to the threat they pose to its nest. This ability suggests that these birds are both intelligent and very flexible in their behaviour – two traits that must surely stand them in good stead in the urban jungle.
It obviously benefits an animal to be able to distinguish between threatening and harmless species, but discriminating between individuals of the same species is a much more difficult task – just think about how difficult you would find it to tell the difference between two mockingbirds by eye.
Levey worked with 24 pairs of mockingbirds that had taken up residence on the university’s campus. Hundreds of people walk within five metres of their nests every day and elicit absolutely no reaction. To simulate a greater threat, Levey asked one of his colleagues to approach the nests of birds with fresh clutches, and touch their rim for 15 seconds. When faced with such intrusion, mockingbirds will typically react by rallying from the nest, making alarm calls and diving aggressively at the trespasser.
Immunity to viral infections sounds like a good thing, but it can come at a price. Millions of years ago, we evolved resistance to a virus that plagued other primates. Today, that virus is extinct, but our resistance to it may be making us more vulnerable to the present threat of HIV.
Many extinct viruses are not completely gone. Some members of a group called retroviruses insinuated themselves into our DNA and became a part of our genetic code. Indeed, a large proportion of the genomes of all primates consists of the embedded remnants of ancient viruses. Looking at these remnants is like genetic archaeology, and it can tell us about infections both past and present.
When retroviruses (such as HIV, right) infect a cell, they insert their own DNA into their host’s genome, using it as a base of operations. From there, the virus can pop out again and make new copies of itself, re-infect its host or move on to new cells.
If it manages to infect an egg or sperm cell, the virus could pass onto the next generation. Hidden inside the embryo’s DNA, it becomes replicated trillions of times over and ends up in every single one of the new individual’s cells.
These hitchhikers are called ‘endogenous retroviruses‘. While they could pop out at any time, they quickly gain mutations in their DNA that knocks out their ability to infect. Unable to move on, they become as much a part of the host’s DNA as its own genes.
In 2005, a group of scientists led by Evan Eichler compared endogenous retroviruses in different primates and found startling differences. In particular, chimps and gorillas have over a hundred copies of the virus PtERV1 (or Pan troglodytes endogenous retrovirus in full). Our DNA has none at all, and this is one of the largest differences between our genome and that of chimps.