All the Homo naledi skeletons in Rising Star Cave are female, and that probably didn’t happen by accident.
In 2013, a team of anthropologists led by Lee Berger unearthed the remains of more than 20 small-bodied hominins (ancient relatives of humans), all 335,000 to 236,000 years old, from the Rising Star Cave System in South Africa. Excavations at Rising Star have sparked debate about whether these little hominins had all ended up in the caves by tragic accident, or whether they’d been carefully placed there by other members of their enigmatic species, dubbed Homo naledi.
Now there's a plot twist that may speak to how the remains got there: All of the hominins in Rising Star are female, at least according to the proteins in their dental enamel.
For decades, biologists thought that early tetrapods, ancient vertebrates that started conquering the land over 300 million years ago, developed like modern amphibians—beginning their lives as purely aquatic tadpoles and then metamorphosing into terrestrial adults. “A lot of that comes from this old ‘scala naturae’ idea that you had fish that evolved into the next stage up, which were amphibians, and then amphibians evolved into the next stage up, which were reptiles that evolved into birds and mammals,” said Jason Pardo, a research associate at the Field Museum.
We’ve never had evidence that early tetrapods had an amphibian lifestyle; we have assumed it because it made intuitive sense. “It’s easier to make the transition from water to land if you’re already making that transition as part of your life cycle,” Pardo said. But now, a new Science study that Pardo co-authored with Arjan Mann (the Field Museum's assistant curator of early tetrapods) shows our most basic assumptions about the first tetrapods that started living on land might be wrong.
Baby monsters
The researchers' study focused mainly on embolomers, an extinct group of large predators that lived roughly 300 million years ago. Embolomers looked like a cross between a crocodile and an eel, with large skulls full of sharp teeth, followed by long, eel-like bodies. It had short, stocky limbs adapted mainly for paddling in water, but also capable of powering brief, clumsy excursions on land. They are thought to be one of the first vertebrates that made a partial transition from an aquatic to a terrestrial lifestyle. These animals could reach over three meters in length, but to understand the very beginning of their life cycle, scientists focused on examining some of their centimeter-scale babies.
Last week, we looked at a new study of the origin of complex cells, one that showed that our ancestors' genomes were pieced together from bits and pieces of multiple species. It put a spotlight on a phenomenon called horizontal gene transfer, in which a gene from one species is incorporated into the genome of a distantly related species. The frequency of horizontal gene transfer means that, in addition to the neatly branching trees that relate species by common descent, there are small threads connecting distant branches of the tree of life.
It's easy to see why horizontal gene transfer would be common among microbes. They often live in complex communities that are likely awash in the DNA of dead and damaged cells. Plus, bacteria and archaea lack a membrane between their DNA and the rest of the cell, making it easier for environmental DNA to find its way to the genome.
However, a new study this week shows that horizontal gene transfers are remarkably common even in multicellular animals. And it does so by examining the genomes of multiple cockroach species, which have had bits of bacterial DNA for millions of years.
We tend to view ourselves and the complex cells that build us as a distinct branch of the tree of life from the compact, seemingly featureless cells of bacteria and archaea. But we've found that our genome is actually a hybrid, a mish-mash of genes from bacteria and archaea, along with some that have evolved in our own lineage.
Scientists gradually settled on a simple explanation for this: the first complex cells were the product of a fusion between archaeal cells and bacteria, with the bacteria ultimately evolving into the mitochondria, a chemical-power-generating structure that still retains a bit of its own genome. Over time, many of the other bacterial genes were transferred to the nucleus of what was becoming what we now call a eukaryote, intermingling with the archaeal genes there.
But a new study has taken a careful look at some of the genes shared by all eukaryotes and comes to the conclusion that the reality is a little more complicated and that there were several waves of gene transfers from bacteria. The big picture of a merger between bacteria and archaea is still right, but it was only part of a picture where gene transfers among species were commonplace.
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