Earliest animal fossil was the size of a grain of rice.

During the Cryogenian, a tiny organism began to spread across the shallow seas.  750 to 550 million years ago, these tiny organisms, just  0.3 to 5mm in length became a dominant force in the ancient oceans.  But despite their size, their importance cannot be underestimated.  For they are the oldest animals to be discovered, predating any other animal fossil by 100 to 150 million years.

Scanning Electron Microscopy of Otavia antiqua taken from the Kuibis Subgroup of the Zaris Formation near Kliphoek in southern Namibia.

Given the name Otavia antiqua and found in the oldest rocks in Namibia, they show that animal life has it roots much further in the past then had been previously expected based upon earlier fossil evidence.  They thrived during a time known as the Cryogenian, the second period of the Neoproterozoic that lasted from of 830 to 635 million years ago.  A period that gains its name from two events where global temperatures plummeted causing the most extensive glaciation events in our planets history, known as the Snow Ball Earth events. During these occurrences glaciation covered most, if not all of the planet, if the hypothesis is correct that is.

The earliest Otavia a. fossils predate the first of the two predicted glaciation events, known as the Sturtian.  Fossils persist up until till the end of Precambrian, where the rise in numbers of complex animals truly begins.  They were simple animals, believed to be Poriferans, sponges.  They have most of the features shared by modern sponges with the exception of spicules (shards of hardened material that are produced by the sponge for structural support), though they may have possessed these too but due to their tiny size and ancient age, little trace is left of them.

They had a very simple body plan.  Most were ovoid to globular in appearance with three distinct sections.  The outer layer was covered in many small pores, known as ostia with a size of 5 to 20 microns in diameter.  These let water into the second section, the peripheral labyrinth.

This section most likely allowed for the start of absorption of nutrients via consumption of algae and bacteria, making Otavia a simple, if sessile, predator.  This section lived up to its name with many winding passages, allowing for the most surface area, increasing nutrient absorbtion as well as gas exchange.

Water would then pass into the central chamber that made up the majority of the internal space of the animal.  This spongocoel (called a paragastric chamber in the paper) would have, most likely, been lined with choanocytes, just as the peripherial labyrinth would have been.

These cells, common to all sponges, beat the water with their flagellum to create a current to aid in respiration and bring in new nutrients.  Water is then evacuated from the spongocoel through the largest opening, known as the osculum.  In Otavia, this opening would have attained a size of many tens of microns in diameter in the larger individuals. 

Otavia also features a trait common to most sponges, mineralization.  While the exact mineral is still in question, Calcium Carbonate or Dolomite are the two most likely suspects based on the composition of Otavia fossils.  This shows that by the time these tiny sponges evolved, they had already begun mastering the process of depositing minerals within their internal structure to provide a more rigid structure.  In essence, the earliest evidence for something akin to a skeleton.  While non-living and quite different from the skeletons that would emerge later within the animal kingdom, it still sets a precedent.

The genus name for these tiny creatures comes from the rock structure where the most specimens were discovered, and by chance, the oldest known individuals.  They were removed from a black limestone portion of the Otavi Group of Namibia.  Combined with the other rock structures that Otavia specimens have been recovered, over a thousand fossils have been recovered, suggesting that they were an incredibly successful group.

While the emergence of these creatures predates the occurrence of any other animal fossil by at least a hundred million years, the discovery was not totally unexpected.  In what is sure to become yet another famous success story in the field of molecular biology, the first animals were predicted to have a common ancestor at exactly the time period when Otavia dominated.

Using a process known as a molecular clock, genomes of various organisms are compared to one another to look for both commonalities and how far apart various genes are.  By understanding just how differences in genes arise through evolution, we can look into the deep past and predict when organisms shared their last common ancestor.  These predictions have been supported by fossil evidence time and time again.  The emergence of Otavia at precisely the time when the last common ancestor of all metazoans was predicted to have lived, suggesting that it may very well be the ancestor of all modern animals, or lived along side it.

This suggests that we owe our very existence to an organism hardly the size of a grain of rice.  A rice grain that survived some of the most hostile climatic changes that our planet has ever experienced, allowing for the vast array or animals seen today to differentiate and spread into every ecosystem on the planet.

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References:

Brain, C., Prave, A., Hoffmann, K., Fallick, A., Botha, A., Herd, D., Sturrock, C., Young, I., Condon, D., & Allison, S. (2012). The first animals: ca. 760-million-year-old sponge-like fossils from Namibia South African Journal of Science, 108 (1/2) DOI: 10.4102/sajs.v108i1/2.658