In the 3.5 billion years since life first evolved on Earth, it was thought that once-free-living bacteria had merged with other organisms on just three occasions, making this an exceedingly rare evolutionary event. Now, a fourth example has been found, in a single-celled alga common in the oceans.

These algae were thought to “fix” nitrogen – convert atmospheric nitrogen into useable ammonia – with the help of a bacterium. Tyler Coale at the University of California, Santa Cruz, and his colleagues have now shown that this bacterium has evolved into a new cellular structure, or organelle.

It is the first known nitrogen-fixing organelle, or nitroplast, says Coale, and could be the key to the success of these algae. “It appears to be a successful strategy for them,” he says. “These are very widespread algae. We find them all over the world’s oceans.”

Our results demonstrate that sperm whale vocalisations form a complex combinatorial communication system: the seemingly arbitrary inventory of coda types can be explained by combinations of rhythm, tempo, rubato, and ornamentation features. Sizable combinatorial vocalisation systems are exceedingly rare in nature; however, their use by sperm whales shows that they are not uniquely human, and can arise from dramatically different physiological, ecological, and social pressures.

These findings also offer steps towards understanding how sperm whales transmit meaning. In some organisms with combinatorial codes, such as honey bees (Apis sp.), the constituent features of the code transparently encode semantics (e.g., direction and distance to food sources). Further research on sperm whale vocalisations may investigate if rhythm, tempo, ornamentation, and rubato function similarly, directly encoding whales’ communicative intents. Alternatively, one of the key differentiators between human communication and all known animal communication systems is duality of patterning: a base set of individually meaningless elements that are sequenced to generate a very large space of meanings. The existence of a combinatorial coding system-at either the level of sounds, sound sequences, or both-is a prerequisite for duality of patterning. Our findings open up the possibility that sperm whale communication might provide our first example of that phenomenon in another species.