How Did That Evolve?

The amazing diversity of organisms in the world never fails to inspire wonder and amazement in people, regardless of scientific inclination. It also helps to inspire Oxbridge interviewers with a variety of perplexing interview questions that challenge students to explain how some characteristic features came to be. An old favourite is “As bats and birds both have wings and can fly, this suggests that they share a common ancestor – discuss”. I’ll return to this question in my next blog to allow you to develop your own answer, but in the meantime let’s discuss shells…

Shells are interesting because they appear so frequently in our fossil record, their composition lending itself to preservation through fossilization. However, while this makes it relatively easier to track the appearance and change of organism’s shells over time, it is vastly more difficult to track changes in organisms that don’t have shells! Indeed, it is hard to believe it but today’s squishy creatures like octopus or squids used to have a shell. Unfortunately, being soft-bodied means these animals rarely leave anything behind after they pass which makes tracking their lineage exceptionally difficult. But using a combination of genetic sequencing and fossil remains — few but valuable — researchers at the University of Bristol in the United Kingdom were able to establish when the cephalopods’ last shelled ancestors used to live. According to the results, squids and octopuses lost their shell during the Jurassic and Cretaceous periods.

According to the research, cephalopod ancestors started losing their shells around the Mesozoic Marine Revolution, which was one most important faunal changes in life’s history. Because of pressure from extremely adapted Mesozoic marine predators, many marine invertebrates had to develop new defenses or risk becoming wiped out. This meant ditching the shell which made it difficult to jet away in the face of threats. As this arms race escalated, it was then found that there was a clear demise of the number of squid and octopus ancestors which had heavy internal shells beginning with 160 million years ago. Up until 100 million years ago, some of these cephalopod ancestors gradually lost their hard shells in favor of more squishy body parts familiar today which helped the animals both catch prey and evade predators easier.

The researchers hypothesized that by having a reduced internal skeleton compared to their ancient relatives, the modern squids and octopuses could compress their body and more efficiently jet away leaving a baffling cloud of ink with the attacking predator. Before the predator realises what has happened and gains clear view again, the squid is far out of sight. To support this theory, the key to these findings was a genetic technique which enabled the researchers to build a timeline of different cephalopod ancestor branching in the family tree. After the molecular clock results were compared to fossil records, the two converged to paint the story of when squids and octopuses lost their shells.

Such research highlights how modern genetic techniques can lend themselves to investigating age-old problems! Consider this as you consider the bat and bird conundrum…