The Cambrian explosion may have been fueled by poop.

The first animals on Earth emerged about 600 million years ago, and 60 million years later, those simple early animals evolved in wildly different directions, branching out into a diverse menagerie of complicated new forms and conquering new ecological niches. Known as the Cambrian Explosion, this flurry of rapid evolution produced most of the animal phyla and ecosystems we see around us today.

And it may have taken off because some of those early animals evolved a bizarre new ability: pooping. According to a recent study, the first poo pellets helped spread nutrients to deeper parts of the ocean, fueling the evolution of bigger animals in more diverse ecosystems.

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When the history of our ancestry is written, the fact that we've interbred with some of our closest relatives, the Neanderthals and Denisovans, will have a central role. And it will be tempting to write it as a very tidy story: Once we got genomes from these other groups, it was possible to identify the sequences in our genomes that we shared with them.

But in reality, as scientists started poking large enough collections of data, there were regular hints of some strange ancestry in our genomes. It was hard to pin down, though, at least in part because the 2 percent on average of Neanderthal DNA found in many populations does not guarantee that any two individuals will have the same 2 percent. So having the genomes of those two groups made sense of some things researchers had already been seeing.

But knowing what we do about Neanderthal and Denisovan DNA is now allowing researchers to answer a somewhat different question: Is there anything else? Using recently developed analytical techniques, they find evidence of a third lineage that we apparently interbred with before any modern humans left Africa. Again, there were hints of this earlier, but so far, there has been no genome from a modern human relative to help us understand the details—the source of this DNA remains a "ghost lineage."

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