On the Origin of Animal Species

Animals are one of the most prolific and well-known among the six kingdoms of life, which include plants, fungi, and bacteria. From simple fish to gargantuan elephants, animals have dominated virtually every ecosystem for a significant portion of the Earth’s history. We can see animals in practically every inhabited environment, save for those rare conditions tolerated only by extremophiles. And yet, surprising as it is, we aren’t entirely sure where the animal kingdom came from -- though we have formed theories as to its origins. These theories, which have been made possible by monumental scientific advancements and discoveries of new fossil sites, have given us a glimpse into the strange worlds that preceded our own.

To understand the rise of animal life, we must first travel to the Ediacara Hills of South Australia. Here, a rich variety of fossilized organisms have been discovered and radioactively dated -- uncovering a catalyst period between the Proterozoic and Phanerozoic eras. Known as the Ediacaran, named so after the place of origin of these fossils, this time period is estimated to have lasted from around 638 million years ago to roughly 541 million years ago -- being the direct predecessor to the famous Cambrian Explosion. In this geologic period, a strange mixture of life forms ruled the prehistoric seas.

Deep in the sedimentary rocks of southern Australia lies the secret of our origins as animals.

Among them were widespread bacterial mats, made from countless single-celled organisms existing within single colonies. These mats, which served as the launching point for multicellular life, was a major food source within typical Ediacaran ecosystems. In fact, these mats often form fossils known as stromatolites -- pillars of fossilized cyanobacteria whose generations lived on top of their dead relatives. Though most of the Earth’s photosynthetic life existed in these colonies, much of it took the form of single-celled algae, which spread throughout the oceans as the newest autotrophic clade within the ocean. But the Ediacaran isn’t important just because it happened to support algal life.

A field of stromatolites found in Hamelin Pool of Shark Bay, Western Australia. Many still support living cyanobacteria, and can thus be considered living fossils of a unique kind.

Throughout the 20th century, most of the fossils excavated from the Ediacara Hills were thought to be related to today’s marine plants. However, with further research scientists concluded that these were actually some of our earliest ancestors. Frond-like fossils, once thought to be remnants of prehistoric seaweeds, are now thought to be precursors to modern sea pens -- distant relatives of jellyfish and sea anemones. Meanwhile, fossilized sediment trails allowed the scientific community to identify some of the first mobile animals -- many of which are the first known examples of bilateral symmetry.

A well-preserved fossil of Dickinsonia, a proposed genus of extinct worm-like organisms with symmetry along the length of their bodies.

The discovery and recontextualization of the Ediacaran fossils allowed scientists to classify the late Ediacaran as an evolutionary radiation -- a period of time across which a wide variety of species develop and diversify. This event, now known as the Avalon Explosion (575 mya to 541 mya), is what we now believe to be the origin of the animal kingdom, giving rise to those lucky few taxa that would carry the torch of life into the first period of the Phanerozoic Era -- the Cambrian.

Most paleontology enthusiasts know about the Cambrian Period. It was the first period of the Paleozoic Era -- a distant age where bizarre creatures ruled the Earth, from eagle-sized insects to sail-backed synapsids. But what makes the Cambrian so special -- aside from its catchy name -- is what it provides for the story of life. With the discovery of the Burgess Shale, a large outcrop of sedimentary rock in the Canadian Rockies of British Columbia, paleontologists uncovered the next stage in our understanding of prehistoric fauna. Building on the milestones of the Avalon Explosion, the Cambrian period was a time where the fauna of yester-eon gave rise to the major animal phyla of today. Many of the categories of animal life we know of -- chordates, arthropods, and annelids -- developed their body plans during the Cambrian Explosion -- the next evolutionary radiation in Earth’s history. It was here that the soft-bodied pseudo-worms branched off in a variety of directions. Most of the species that dominated this period were arthropods -- invertebrates with tough, chitinous exteriors to protect themselves from predators. Today, we see arthropods in the form of insects, arachnids, and other taxa -- but over 500 million years ago, there were far more fascinating body plans.

Structure of the chitin molecule, a polymer of the monosaccharide N-acetylglucosamine. It is a primary component of fungal cell walls, the radulae of mollusks, and the exoskeletons of arthropods.

The trilobites, a taxon whose fossils can be found almost anywhere on the planet, were some of the most abundant creatures ever to exist. With their novel exoskeletons and ability to scurry and burrow, trilobites were both hunters and scavengers, feeding on detritus and occasionally other invertebrates. Even with these defenses, however, trilobites were a common food source for Earth’s first superpredators.

With intricate fossils such as that of the trilobite, scientists have managed to piece together the anatomy of Earth's ancient inhabitants – starting with some of the first arthropods.

Dinocaridida, a proposed taxon of extinct marine arthropods, comprises a large swath of organisms reminiscent of shrimp and lobsters. Like the trilobites, they had an articulated exoskeleton, allowing them to maneuver with ease through the Cambrian ocean. They used prehensile, proboscis-like appendages to move prey to their jawless mouths, ruling the marine world with a chitin-coated fist.

Because of the wonderful detail within which organisms were fossilized in the Burgess Shale, remnants of organisms like Anomalocaris canadensis (to the left) can be reconstructively modeled to get a better view of what it might have looked like (to the right).

Among the arthropods, however, were the lobopods -- a taxon of worm-like organisms. With soft, fleshy appendages used to crawl across the ocean floor, the lobopods often resembled millipedes in appearance, using small tendrils to scoop bits of edible material into their mouths. Slow as they may be, the lobopods often came with defensive spikes along the length of their body. These were discovered when scientists first uncovered fossilized lobopods, mistaking them for stilt-like legs. At first, they thought that primordial lobopods tiptoed across the seabed on rigid protrusions -- but soon they found that, in fact, they were looking at the creatures upside down, confirming their other pair of more flexible extensions to be the true legs. Because of their confusing appearance, the scientific community named a specific group of lobopods Hallucigenia -- a common taxon of lobopods that made up a significant portion of the Cambrian ecosystem.

Ever since it evolved, without stopping once, it kept moving forward.

A third monumental evolution in the Cambrian Explosion was that of the first vertebrates -- small, hagfish-like creatures without the jaws we commonly associate with the bony fish of today. These organisms, known collectively as the first craniates -- animals with distinct heads and notochords, which are backbone-like rods of cartilage. Though far simpler and more vulnerable than the vertebrates of today, these plucky fish would ultimately go on to give rise to every craniate living today -- from Tyrannosaurus to Paraceratherium to humankind.

Like their arthropod cousins, early vertebrates like the jawless fish Pikaia gracilens were fossilized in stark detail, much to the excitement of the scientific community.

Though only a small chapter in our story, the Avalon and Cambrian explosions help us to understand precisely where animal life is thought to have branched off from other eukaryotes. With further investigation, we may learn even more about this distant age and the many surprises it has to offer.