A new paper in Nature by Jian Han et al. (reference and free link below; one of the coauthors is Simon Conway Morris, of Burgess Shale fame) describes the earliest known deuterostome: that superphylum of animals in which the blastopore (the first opening into the central part of the embryo) becomes the anus, and the second the mouth. (“Deuterostome” means “mouth second”). This is in contrast to the group of protostomes, in nearly all of which (there are exceptions) the first opening becomes the mouth, and the anus develops later at the other end.
Deuterostomes include all chordates (including vertebrates like us), echinoderms, and “hemichordates” (acorn worms and graptolites). Protostomes include everything else, including segmented and unsegmented worms, molluscs, insects, rotifers, and so on. Here’s a diagram showing the developmental difference, and a second diagram showing the big divisions of life.
The first deuterostomes, previously dated at 510-520 million years ago, have now been supplanted by Han et al.’s finding of a tiny creature (about 1.2 mm across) in central China, with the sediments dated to 540 million years ago. Although it has a huge mouth and no anus, it still shows features suggesting it was an early deuterostome, one that lived among the sand grains of the sea floor. The authors named it Saccorhytus coronarius, part of a new group called the Saccorhytida.
But first, some journalistic errors. If this creature was an early deuterostome, it would be one of the first creatures from the lineage that led to humans (and other chordates as well as echinoderms) after that lineage split from the lineage leading to protostomes. But that does not make it “humans’ oldest known ancestor,” as is blatantly (and erroneously) indicated by the title below.
That title is bogus! For all living and extinct species, including humans, had an ancestor that was much older than 540 million years—the “last universal common ancestor” (LUCA) of all creatures, which probably lived a bit over 4 billion years ago. The title above (click screenshot to go to link) was in fact from a press release by St John’s College in Cambridge, where Conway Morris works. How could they get it so wrong?
At any rate, that error seems to have been picked up by several other journalists. Here are two examples:
I like this one from LiveScience; it’s not only a bit more accurate, but funny. Actually, it’s not completely true, for this species didn’t have to be a human ancestor itself, for it could have gone extinct without descendants, like some of the early robust hominins. All its deuterostome ancestry shows is that it evolved after the split between the protostomes and the deuterostomes.
But on the paper, which I’ll summarize briefly. Han et al. report finding 45 of these creatures, with the reconstruction in color a couple of photos down. Here’s a scan of the anterior (front) part of the fossil itself, showing its big gob:
The most prominent feature of these fossils is the large mouth opening, clearly seen above, which is surrounded by several rows of papillae that may represent sensory organs. Each side of the body also bears four cones (8 total; you can see four above the mouth in the photo above). Han et al. suppose that the cones could have been used to expel water and waste, though they’re not sure.
These creatures were tiny, as I said, and were examined by both electron microscopy and CT scanning. Here are some photos; note the scale bars (a μm, or one micron, is one millionth of a meter, or one one-thousandth of a millimeter):
But if these are deuterostomes, with the mouth forming after the anus, where’s the anus on these things? They don’t have one! The authors explain its absence this way:
Early deuterostomes have a through gut, so the apparent absence of an anus in Saccorhytus could be secondary, as in brachiopods and ophiuroids. [That is, its ancestors could have had an anus but lost it.] It remains possible, however, that this feature was inherited from more primitive bilaterians, possibly linked to the acoels and xenoturbellids.
But if there’s no anus, how can they call this an early deuterostome? It turns out that the small creature has other features that link it with the deuterostomes, as shown in the phylogeny below derived from several characters (the bootstrap support isn’t all that high). In particular, they show that Saccorhytus bears resemblances to “vestulocystids,” or early echinoderms, which are clearly deuterostomes. These features, also studied by Conway Morris and his colleagues, include truncated cones on the body, a convoluted anterior part of the body, and “well developed radial ribs.”
Here’s the phylogeny showing the new species (in red) falling out with the deuterostomes:
Here’s a figure from a 2004 Nature paper showing the truncated cones (A and C) in some vestulocystids:
Finally, where the fossil was found and the appearance of the sediments (it must have been hard to spot these!):
The authors conclude that our earliest deuterostome ancestosr might well have been tiny and lived among sand grains of the sea floor; they were “meiofaunal”, meaning small bottom-dwelling animals that inhabit the sediments. They also conclude that since these species could have been very tiny, we may have missed even earlier appearances of deuterostomes in the fossil record. Finally, they conclude that respiration occurred through the body surface, and the presence of pharyngeal arches (structures bearing gill slits), which are present in all modern deuterostomes at some developmental stage, could have evolved later.
Han, J., S. C. Morris, Q. Ou, D. Shu, and H. Huang. 2017. Meiofaunal deuterostomes from the basal Cambrian of Shaanxi (China). Nature advance online publication. doi:10.1038/nature21072