Fossils found in rugged mountainous terrain in Canada's Northwest Territories may give a glimpse at the humble dawn of animal life on Earth - sea sponges that inhabited primordial reefs built by bacteria roughly 890 million years ago.
A Canadian researcher said on Wednesday the fossils, dating to a time called the Neoproterozoic Period, appear to show distinctive microstructures from the body of a sea sponge built similarly to a species living today called the Mediterranean bath sponge, or Spongia officinalis.
If this interpretation is correct, these would be the oldest fossils of animal life by roughly 300 million years.
"The earliest animals to emerge evolutionarily were probably sponge-like. This is not surprising given that sponges are the most basic type of animal both today and in the fossil record," said geologist Elizabeth Turner of Laurentian University in Canada, who conducted the study published in the journal Nature.
The Earth formed more than 4.5 billion years ago. The first life forms were bacteria-like single-celled marine organisms that arose hundreds of millions of years later. Complex life evolved relatively late in Earth's history.
The first appearance of rudimentary animal life has been a much-debated topic in terms of its timing and form. An enigmatic ribbed, pancake-shaped organism called Dickinsonia known from fossils dating to roughly 575 million years ago has been considered a candidate as the earliest-known animal.
Turner said she believes animals evolved much earlier than the present fossil record indicates.
"The existence of a protracted back-history is not surprising, but the sheer duration of it - a few hundred million years - may be a little unexpected for some researchers," Turner said.
When people think of animals, a sponge may not immediately come to mind. But sponges - aquatic invertebrates that live fixed to the sea floor and possess soft, porous bodies with internal skeletons - are among the most successful animal groups.
"They lack a nervous, digestive and circulatory system. They have an amazing water-pumping machine, produced by specialized cells, that they use to move seawater through their bodies to filter-feed," Turner said.
Some sponges have skeletons made of microscopic rods of quartz or calcite. Others have skeletons made of a tough protein called spongin that forms a complex three-dimensional meshwork supporting the animal's soft tissue. The Canadian fossils represent this latter kind, called a horny sponge.
"It is the relict structure of the 3-D meshwork spongin skeleton that is preserved and that is so distinctive," Turner said.
This structure, visible under the microscope, consists of tiny tubes that branch and rejoin to form the meshwork. The body size for the sponge would have been roughly four-tenths of an inch (1 cm). Turner said the sponges appear to have lived in cavities just below the reef surface and in surface depressions.
If these fossils genuinely show a type of sponge, their age would indicate that Earth's first animals evolved before a pair of landmark events usually seen as predating animal life.
One of these was the second of two episodes in the planet's history when the amount of atmospheric oxygen greatly increased, sometime between about 830 and 540 million years ago. The other was a tremendously cold time when Earth may have been encased in ice or at least partially frozen over, sometime between about 720 and 635 million years ago.
The fossils predate by about 350 million years what had been the oldest-known sponge fossils. Turner noted that genetic research indicates that sponges first appeared at approximately the time to which these fossils date.