It is not often that a single, non-experimental observation can undermine our view of what happened hundreds of millions of years ago, but that is what happened 780 m below the surface of the Atlantic, around the Bahamas.
A group of US and Australian marine biologists, led by Mihkail Matz from the University of Texas, were surveying the seabed in a research submarine, when they noticed grooved tracks snaking across the seafloor. The apparent creators of the tracks were dark-green grape-like objects, around 30 mm in diameter. Sequencing of small-subunit ribosomal RNA from these objects showed they were massive single-celled organisms – mega-protists of the genus Gromia, previously known only from the Arabian Sea. These amoeboid organisms consist of a thin spherical layer of protoplasm under a membranous `test' (a kind of shell), with the bulk of the organism's volume being made up of inorganic matter.
For most of us who are unfamiliar with deep sea gromiid testates, the very existence of these blobs is a cause for amazement – they are one of the largest single-celled organisms. But the truly dramatic part of the observation comes from the fact that these organisms leave tracks. Could their ancient ancestors be responsible for some of the oldest trails in the fossil record?
Fossilised bilaterally symmetrical animals (Bilateria), which appear in the fossil record around 542 million years ago and may have appeared 80 million years earlier according to molecular data, are thought to have left sinuous groove shaped traces on the pre-Cambrian seabed that eventually became preserved in the fossil record. However, some of the marks can be found in rocks up to 1.5 billion years old, long before the evolution of Bilateria. These tracks have never been explained, and have led some paleontologists to suggest that Bilaterians, which had been believed to have produced the tracks, appeared far earlier than the Cambrian era.
The discovery of similar traces associated with modern non-Bilaterians raises the possibility that the ancient grooves were made by giant protists instead. If the fossil tracks cannot be unequivocally assigned to Bilaterians, then the earliest date at which we can be confident our distant ancestors appeared will be brought forward to their first appearance in the fossil record, rather than the first appearance of the tracks that had been attributed to them. It might also suggest that the Ediacaran biota, the earliest known complex organisms that precede the animals of the Cambrian explosion, were giant, turgid protists, like Gromia.
There is a catch, however. The researchers did not actually see the mega-protists making the grooves, although the grape-shaped organisms were always seen with their axes perpendicular to the trace, and they had extended pseudopods that may, as in other gromiids, be used to pull the protist along. The authors hypothesise that Gromia creates the grooves by picking up sediment in front of the test and excreting it behind it. They suggest that it is this process, rather than movement itself (the protists have nearly neutral buoyancy), that produces the grooves.
The final factor that facilitates the appearance of the traces is the persistence of the sediments and the extremely weak currents in the area. The authors suggest that the observed tracks – none more than 50 cm long– may have taken months to produce. Catching the gromiids in the act may prove extremely difficult, unless they can be brought to the surface and persuaded to roll in the laboratory. Deciding whether pre-Cambrian trace fossils were left by mega-protists or Bilaterians will be even trickier, but these deep-sea observations have raised an important doubt over what were thought to be the earliest traces of animal life.