Evolution is one of the most robust and comprehensive scientific theories in existence. In fact, in terms of volume of evidence, we have more proof for evolution than we do for many other widely accepted scientific theories like gravity or cells. However, we still don’t understand evolution, like where certain animals fit on the Tree of Life. We call these anomalies phylogenetic mysteries.
Phylogeny is the study of the evolutionary relationships between different groups of organisms. Essentially, phylogeny aims to trace the evolutionary history of all life on Earth based on the principle that all living things share a common ancestor. Scientists show this relationship between organisms using a phylogenetic tree, sometimes called The Tree of Life.
We call it a tree because new organisms appear over time due to environmental pressures while all organisms descended from one ancestor. When a new organism evolves, it splits off from the original group, creating a new branch in the tree. Over many millions of years, this can happen several times. For example, both mammals and birds split off from reptiles in two separate branches. And of course, there are many thousands of species of mammals and birds, so these branches split several times again.
For the most part, nearly all well-documented organisms fit neatly into the tree of life, firmly placed amongst their ancestors. However, for a few species, the picture is much less clear1.
Some organisms refuse to be placed into a neat little box and instead have scientists scratching their heads. These organisms' specific characteristics could put them in multiple groups, so it's unclear which group they belong to.
When scientists learn more about these organisms, the mystery becomes solved by finding more transitional fossils or examining their genetic code. Solving these mysteries is crucial because it tells us where those animals belong and helps us understand how the whole picture comes together.
Let's take a look at the most puzzling phylogenetic mysteries.
Within-lifetime phylogenetic mysteries are mysteries where an organism looks dramatically different throughout different stages of its lifecycle, but scientists haven't witnessed the complete lifecycle. This can lead to a situation where the same species is categorized as two different species. For example, if you were to show someone from another planet a maggot and a fly side by side, they would probably think they are entirely different species. This mystery is typically more familiar with ocean life, which can be much harder to study. For example, for centuries, people were perplexed about where baby eels came from. We only ever saw adult eels. All sorts of strange theories were invented about how eels were made. Some scholars thought they were made from mud and rain! It turns out eels travel enormous distances and have a very complex five-stage lifecycle, so we just never saw baby eels and adult eels in the same place2.
Some species don't change appearance dramatically with age, but rather, sex. Sexual dimorphism (the difference in characteristics between the sexes of a species) is expected in the animal kingdom, but some examples are more pronounced than others. For instance, out of the 46,000 species of spider currently documented, 51% have been studied using only one sex.
One example is Sun spiders, also called camel spiders. Scientists had only studied the males, so when they found the very different-looking females, they believed them to be a different species3.
Some creatures have characteristics that are so strange that scientists believed they must have only evolved once. For example, termites, ants, and bees are eusocial organisms. Eusocial insects live in colonies where members have specialized roles, like workers, drones, and queens, and only some members can reproduce4. Scientists wrongly assumed that this meant termites, ants, and bees must be closely related5, so they were placed together on the phylogenetic tree. However, further research found that termites are not much like ants or bees, and it would be more accurate to call them highly social cockroaches.