We hear often of the Cambrian explosion – the period around 540 million years ago wherein many animal groups first appear in the fossil record – but, contrary to the impression that you might get from certain creationist sources, there was life before the Cambrian. For example the Ediacaran, the Period immediately prior to the explosion, is known for its enigmatic biota, including the disk-shaped Aspidella.
The nature of the Ediacaran biota is of importance: molecular studies – i.e. genetic comparisons – of living animals show that they actually diverged some time before the Cambrian, two periods back in the Cryogenian. If the common ancestors of all living animals lived in the Cryogenian, therefore, then there must also have been animals in the Ediacaran. Some evidence that this is true, therefore, would be quite nice.
But it’s not that simple. All known Ediacaran organisms are soft bodied, meaning that they leave only rare trace fossils (impressions of various kinds) that are difficult to interpret with certainty, but they are generally thought to be multicellular marine organisms. However, a paper published (pdf) around the start of the year contended that Ediacaran fossils from South Australia, which would include Aspidella, were in fact deposited on land and may even be more likely to be “lichens and other microbial colonies of biological soil crust, rather than marine animals, or protists.”
That brings us to a more recent paper, Evidence for Cnidaria-like behavior in ca. 560 Ma Ediacaran Aspidella (closed access, no pdf available). Cnidaria is a broad group of animals which includes corals, jellyfish and sea anemones: the behaviour alluded to is that of an organism living of the sea floor slowly adjusting its position to ensure that it doesn’t get buried by accumulating sediment, which leaves behind what they refer to as “equilibrium traces.” This shows in the minds of the authors that Aspidella did live in a marine environment and was an animal. The ICR’s Brian Thomas, in What Were the First Animals Like?, accepts this conclusion but draws one of his own from it. He opens:
Are Ediacaran “fossils” actually remains of ancient living things, or did simple natural processes generate fossil look-alikes? Correctly identifying these tracks (or traces) matters significantly to those who insist these Ediacaran rocks—which secularists believe to be over 550 million years old—came from a time when Earth’s earliest animal life first appeared. Are these scientists looking at fossils made by the supposed ancestor common to all animals?
This and other passages in his article are confusing – for example, I’m confused as to whether or not Thomas himself is confused or whether he’s just explaining this poorly. You might conclude from the above that the (trace) fossils of Aspidella are in doubt, and might instead be formed by natural processes, but this is not so. Instead, these researchers found other traces which they interpreted as being from the aforementioned adjustments, but first they needed to prove that these were not created abiotically. Thomas explains:
The Geology authors proposed a few natural mechanisms, including fluid pipes and collapsed sediments, that might have explained the formation of Aspidella. However, they could not replicate these Aspidella structures in laboratory experiments, a fact that indicates biological origins.
This was actually a very interesting experiment. From the paper:
Another possible explanation for the vertical structures invokes gravitational sediment collapse into a void. Such voids might have been created by decomposition of the body tissues of the bulb-like portion of the Aspidella-making organism. Gravitational collapse has previously been observed typically to produce a flame-like, downward-pointing profile, and increasingly shallow dips in overlying strata (Buck and Goldring, 2003). To test this further, we conducted experiments in small aquaria, in which mud and sand layers were built up over buried dissolving fluid-filled capsules. The dissolution of the capsules produced voids into which the overlying sediment layers collapsed. The resultant sedimentary fabrics show increasingly shallow dips upward through the sediment pile overlying the position of the dissolved (void producing) structure (Fig. 3F). This is contrary to observations of the Fermeuse structures, which show broadly parallel inwardly dipping laminae, often slightly thickened with coarser grains, and usually culminating in a rounded form with coarse sediment infill. Such vertically stacked meniscate structures are unlikely to have formed by gravitational collapse above a decayed Aspidella body.
In short, they buried some dissolving capsules, representing a decomposing Aspidella which, rather than slowly burrowing upwards, stayed in place throughout its life. They found that the sedimentary structures produced in this process were quite different from what was observed in the rock, ruling out that mechanism. Am I the only one that thinks this sounds like a fun science fair-type experiment?
Continuing with Thomas:
The researchers concluded that Aspidella traces look like animal tracks—specifically, tracks that today’s sea anemones might have made.
More or less. The abstract of the paper says that “equilibrium traces are familiar from the Phanerozoic and are observed in partially buried marine animals such as tube anemones today.” However, they say later: (bolded, in context)
It is even more significant that no large protist has ever been observed to adjust its vertical height in this way. Although a living infaunal xenophyophoran protist is known, it is associated with regular, horizontal, Paleodictyon-like hexagonal tests (Levin, 1994). In contrast, equilibrium traces typically result from life at, or with continuous access to, the sediment-water interface. Equilibration therefore implies complex behavior, involving life at the boundary layer, and the ability to respond rapidly to mild environmental stress induced by sedimentation. Furthermore, since sponges cannot move vertically through sediment, such behavior indicates a eumetazoan trace-maker, able to propel itself through small pulses of fine sediment by muscular contraction. Unlike modern burrowing cerianthids (tube anemones), however, there is no evidence of a lined burrow. The simplest explanation is that Aspidella was a burrowing or facultatively vagile epifaunal animal of cnidarian grade.
The traces, therefore, are similar to that which we might see from anemones, but there are differences also. These are the only mention of anemones in the paper, so I don’t know where Thomas gets the idea that:
Another clue came by comparing Aspidella tracks to sea anemone anatomy. They match.
That’s not particularly important, however. We’ve seen the conclusion drawn by the authors – what did Thomas have to say? After noting that the authors mention how the Aspidella moved by “muscular contraction” (see above quote) and similar, he concludes:
So, right from the start, animals had functioning nerves, muscles, and body shapes integrated with behaviors. In other words, they have always had everything they need to live, just as Genesis asserts.
The clues these scientists uncovered show that creatures still identifiable as living today very likely made the Ediacaran fossil traces. Which idea requires more faith: imagining that 560 million years of mutation and natural selection generated no changes to these mud-inhabiting anemones or understanding that God fully outfitted anemones right from the start?
What were the first animals like? They were like the ones we see today.
There are many problems here. For one, he has jumped to the conclusion that not only does Aspidella move in a manner similar to sea anemones, but it is a sea anemone – I shouldn’t have to explain just how large of a conclusion jump that is. Secondly, having “nerves, muscles, and body shapes integrated with behaviors” is basically the definition of animals, or at least those that aren’t sponges. It is completely unsurprising that fossil animals would have these features, as if they didn’t they could not be animals, or if they still were we would have no way of knowing.
A final thing to remember is that, as I said earlier, Aspidella is from the Ediacaran, while the common ancestor of all animals is predicted to be from the Cryogenian. Therefore, nobody thinks, as Thomas claims in his first paragraph, that we are looking at fossils “made by the supposed ancestor common to all animals.” Nor should we expect Ediacaran animals to lack the features common to all animals, as the common ancestor, which lived earlier, should have had all of them.
Maybe one day we’ll find the Cryogenian animals, and catch a glimpse of how they came to be. For now, however, we will have to make do with an Ediacaran animal, which is interesting enough.