September 2011

Back in the Jurassic, dinosaurs may have dominated terrestrial ecosystems, but they were not alone. Scurrying around their feet and clinging to the trees above them were the fuzzy ancestors of their successors. When most of the dinosaurs perished, the surviving mammals diversified into the dinosaurs’ niches, where they remain today. Last month, scientists reported on the discovery of a fossil mammal from China that would have lived alongside the dinosaurs and that, at 160 million years old, represents one of the earliest mammals known. Juramaia sinensis may not have been much to look at — a furry rodent-like animal just a few centimeters long — but it’s made big news. Is this tiny critter really the “missing link” in mammal evolution and the “mother of us all,” as many articles have suggested?

Where's the evolution?

To understand Juramaia sinensis‘s place on the family tree of mammals, take a look at this phylogeny. It shows clades of monotreme mammals (mammals that lay eggs, like the platypus — in red), placental mammals (mammals that give birth to live young and nourish them through a placenta, like humans — in orange), and marsupial mammals (mammals that give birth to live young and nourish them in a pouch, like kangaroos — in blue). It also shows relationships to their extinct relatives, represented by dots that mark their fossils. Juramaia sinensis is represented by a yellow dot in the mid-Jurassic.

You can see that Juramaia is more closely related to placental mammals than to marsupials or monotremes. Many different lines of anatomical evidence were used to reconstruct this phylogeny and support the idea that Juramaia is at least a close cousin of placental mammals. For example, Juramaia has three molars and five pre-molars — like placentals, but unlike marsupials which have four molars and three pre-molars. So Juramaia is a very early relative of placental mammals (it predates all other placental-type mammal fossils by at least 35 million years), but is it really “the missing link” in mammal evolution?

First, let’s examine the idea of a “missing link.” When we describe something as missing, it usually implies that the item is supposed to be present, but for unknown reasons, no longer is. In this way, the concept of a missing link in evolution is flawed. Because fossilization is a chancy process and relatively unlikely, biologists expect that many intervening steps of an evolutionary transition (e.g., the divergence of placental and marsupial mammals) will not be recorded as fossils. Because we know why so-called missing links do not make an appearance in the fossil record and because we expect this to be the case, missing links aren’t technically missing at all.

Before the discovery of Juramaia, scientists knew that placental and marsupial mammals had diverged, knew much about when and how they diverged, and had even discovered many early mammals from the Cretaceous and Jurassic (like Eomaia scansoria, shown in green above). Juramaia simply filled in more details of this evolutionary transition. It is most accurate to say that Juramaia is an ancient mammal with many transitional features that help us understand how modern placental mammals evolved from their ancestors.

Juramaia is not a “missing link,” but is the species at least our direct ancestor — the “mother of us all,” as many articles have advertised? Possibly, but most likely, Juramaia is simply closely related to that ancestor. The issue at stake is illustrated in the phylogenies below. Phylogeny A represents the hypothesis that Juramaia is the ancestral placental mammal, and Phylogeny B represents the hypothesis that Juramaia is a close relative of that ancestor. The odds of discovering the direct ancestor of all placental mammals is slim for at least three reasons: (1) only a small percentage of organisms actually become fossils, (2) we discover only a tiny fraction of those organisms that do become fossils, and (3) evolution produces a plethora of dead-end lineages along its winding path. For example, when the next oldest fossil placental (Eomaia scansoria) was discovered about 10 years ago, it too was heralded as an ancestor of all placental mammals. However, the discovery of additional fossils and new analyses make it clear that Eomaia is simply a twig on the branch of the tree of life that gave rise to placental mammals, as shown on the first phylogeny above. Juramaia may be the same.

Whether Juramaia is our direct ancestor or a close cousin of that ancestor (in familial terms, that’s the difference between a great-grandmother and a great-aunt), Juramaia improves our understanding of early mammal evolution in many ways. Perhaps most importantly, the new fossil has helped us home in on the timing of the split between placental and marsupial mammals. Before the discovery of Juramaia, the earliest known fossil relatives of placentals and marsupials dated to around 125 million years old. That meant that, since each lineage clearly existed as a distinct entity 125 million years ago, the divergence of the two must have happened sometime before then. But how long before?

One way scientists narrow in on such dates is by studying the genetic sequences of living placental and marsupial mammals, and working backwards to figure out how much time would have had to have passed for the two sequences to have diverged from each other so much. (This relies on a concept known as the molecular clock.) Such DNA-based estimates had suggested that the marsupials and placentals split over 150 million years ago — but we had none of the sort of fossils we’d expect from this time period if the split had occurred so long ago … until Juramaia was discovered, that is! At 160 million years old, Juramaia fits perfectly with what we would expect to observe if the DNA estimates of the placental/marsupial split were correct. Early relatives of placental mammals, like Juramaia (ones that clearly evolved after placentals and marsupials split), were around more than 150 million years ago. Both pieces of evidence lined up and pointed to a placental/marsupial split sometime between 160 and 180 million years ago.

Juramaia may not be the fabled (but misleadingly named) “missing link” or our great-great-grandmother, but the tiny creature does hold important clues about the evolution of the mammalian branch of the tree of life. It suggests that many of our ancient mammalian relatives may have been climbing around in the trees, instead of dodging the tramping feet of dinosaurs. It spotlights the mid-Jurassic as the context in which the evolutionary history of pouch-bearing marsupials and placenta-bearing mammals began to diverge. And it reminds us how many different lines of evidence we need to put together a complete picture of life’s history on Earth.