Salps are translucent creatures of the ocean that look like the pods of jet aircraft, taking in water at the front and expelling it from the back, extracting particles from the seawater in between. Their humble lifestyle belies a sex life of great complexity and delicacy, in which generations alternate between sexual and asexual phases. But salps have a claim on our attention apart from the picturesque – they are our distant relatives.

The vertebrates (the animals with backbones) include ourselves, as well as most of the animals with which we familiar, from catfish and dogfish to cats and dogs. But vertebrates comprise just one branch (albeit the most visible) in a larger group of creatures, the chordates. These include a shy, fish-like creature called the amphioxus, an sea animal that looks like an animated anchovy fillet. More distant relatives include the tunicates, a large group of several thousand species that includes salps, which swim freely; ascidians, which have free-swimming larvae but spend their adult lives rooted to one spot, and other creatures.

The chordates are united by certain characteristics, such as the possession, at some part of their life cycle, of a hollow, tubular nerve cord (the equivalent of the spinal cord), supported by a rod of stiff tissue called the notochord. This structure, which gives its name to the group, is the rudiment of what, in vertebrates at least, becomes the backbone. But the thing that distinguishes vertebrates from other chordates, and all other animals, is a distinct 'head', bearing complex sense organs such as eyes and ears, powered by a large and complex brain. But where did this brain come from?

Lacalli, Holland and their colleagues have been on the track of the origins of the brain for some years. Although tunicates and the amphioxus are, it must be said, only modestly endowed in the brainpower department, they contain structures that correspond with vertebrate brains, at least in part.

The amphioxus looks totally headless – the notochord extends right to the front of the animal (in vertebrates, it stops at the back end of the skull) and the animal is pointed at both ends ('amphioxus' means just that – pointed at both ends). But in a landmark study in 1994, Lacalli and colleagues mapped the nerve cells in the front end of the amphioxus nerve cord to show that it corresponded with the vertebrate brain. The amphioxus has a head, but it is so modest that you'd hardly know it was there.

Lacalli and Holland have now done the same job on specimens of the salp Thalia democratica. Baby salps have a rudimentary hollow nerve cord that shortens and bunches up into a globular 'brain'. Later, a single eye appears on top of the brain, like a cherry on a cake. Detailed comparison of the salp brain with that of the amphioxus and vertebrates suggests that it corresponds with the mid-section of the vertebrate brain, involved in visual processing, and the source of some motor neurons.

Salps are arguably the most primitive living chordates. Where did they get their brains from? Looking at more distant relations of the chordates – the echinoderms, or spiny-skinned animals such as starfishes and sea urchins – Lacalli and Holland see neural structures in the larvae of starfishes whose organization has some resemblances with the salp brain, and, by extension, our own.