The diet of our ancestors: what tapeworms tell us about human evolution
Most of us don’t think about tapeworms much. They’re uncommon in the developed world, and kind of gross. But it turns out the little freeloaders have something to tell us about human evolution. It has to do with when they started infecting humans.
Before we go into that, let’s talk about how these parasites make a living. There are three species of tapeworm that have humans as definitive hosts (that is whose adult form lives in humans). Taenia solium, shown in the picture below is one of them.
The worms attach to the wall of a person’s small intestine via those hooks you can see on the right. Once there, they absorb nutrients potentially growing to be many meters long. All the while, they are producing eggs. These are carried out into the environment in the person’s stool and get ingested by another animal, called the intermediate host.
In the case of T. solium the intermediate host is a pig. Once inside a pig, the eggs hatch, break out from the intestine and find their way into various organs and tissues. In these locations, they develop into small cyst-like structures and basically wait. If a human then eats the pig, especially without cooking it properly, they get into that person’s gut.
At present, the intermediate hosts for the three human-infecting species are domesticated animals. Because of this, when researchers first considered the question of how tapeworms started infecting humans, they assumed that it happened when we domesticated these animals. The idea was that the ancestors of pigs and cows had these worms, and they got into humans when we domesticated (and ate) them. This would mean tapeworms started infecting us sometime in the last ten thousand years.
However, when researchers looked more carefully at the relatedness of various tapeworm species they found that this idea is wrong.
The image below is a phylogenetic tree, representing the relationships between a number of tapeworm species. The species are listed on the right. The ones in bold have humans as their definitive host — this includes T. solium which we mentioned before, and two others, T. asiatica and T. saginata. The rest of the species listed have various mammalian carnivore species as definitive hosts.
Like all phylogenetic trees, this tree tells us a story about history. First of all, it tells us how the living species of tapeworms are related to each other. The green dots represent common ancestors. For example, the green dot all the way on the left represents the most recently living common ancestor of all tapeworm species in the tree. As we move right from that dot, we go forward in time. The branches (in black) trace lineages descending from that ancestor. As we go, we pass additional green dots, which represent common ancestors for smaller subsets of species.
One thing the tree shows us is that two of the species that infect humans, T. asiatica and T. saginata, are sister species. They are more closely related to each other than they are to any other tapeworm species. We can see this because they share a common ancestor with each other (that green dot right next to them) which they don’t share with the others.
This leads us to a further inference about history. Because both T. asiatica and T. saginata use humans as definitive hosts, it is likely that their common ancestor also used humans as a definitive host. This reasoning is based on parsimony. Switching from one host to another is an unlikely event, and we prefer historical scenarios that require fewer such events. The simplest scenario for T. asiatica and T. saginata involves entering humans sometime before their most recent common ancestor. This would require only one host transfer event. If they entered after the common ancestor existed, it would require two events, which is less likely. We have added orange highlighting to the tree to show where the ancestral lineage likely started using humans as definitive hosts. This seems to have happened twice in tapeworms. Once on the lineage leading to T. asiatica and T. saginata (before their common ancestor). And another time on the lineage leading to T. solium.
Knowing that the common ancestor of T. asiatica and T. saginata already lived in a human is interesting. But when was that? If the original idea that tapeworms got into humans after animal domestication was right, then this would have been recently, sometime in the last 10,000 years.
It turns out we can use molecular sequence data to examine this question. The common ancestor of T. asiatica and T. saginata had thousands of genes. Many of these have been inherited in both T. asiatica and T. saginata. The sequences found in those two living species are not exactly alike, because mutations have occurred in each lineage since the common ancestor’s time. Because such mutations occur at a (relatively) steady rate, it is possible to use the amount of sequence difference between the living species to try to estimate how long it has been since they shared a common ancestor. When this has been done for T. asiatica and T. saginata, researchers have come up with estimates much older than 10,000 years. One study estimated the ancestor to have existed between 780,000 and 1,710,000 years ago. Another estimated it to be between 550,000 and 1,430,000 years ago.
So, the common ancestor of T. asiatica and T. saginata lived a long time ago, hundreds of thousands of years before humans first domesticated cows and pigs. In fact, these estimates are so long ago, that it seems the common ancestor of these tapeworms didn’t live in a modern human at all, but rather in our ancestors.
So how did tapeworms start infecting us? Other tapeworms in the phylogenetic tree infect various carnivores (things like jackals and hyenas). For these worms, particular carnivore species are the definitive host. They use various prey species (such as antelopes) as intermediate hosts. Our human ancestors may have acquired tapeworms when they also started feeding on these same prey species. That is when they started eating meat.
Anthropologists have known for a long time that hunting and eating meat are important in human evolution. This is a major way that modern human hunter-gatherers get food, and it’s clear that extinct species such as Neanderthals also ate large amounts of meat. The interesting thing that tapeworms tell us, is that meat eating started early. The molecular sequence estimates for when the last common ancestor of T. asiatica and T. saginata existed average about a million years ago. That predates both modern humans and Neanderthals, taking us back to an older group of hominins, perhaps Home erectus. It’s amazing that a parasite can tell us about what our ancestors were eating before they even became modern humans!
