11 Evolution and the History of Life: The Origins of Complex Animal Life
The Geological Timescale and the History of Life
The history of Earth is divided into major eons, eras, and periods that track the development of life from its earliest organic structures. The Precambrian stretches from the formation of the Earth approximately billion years ago through approximately million years ago, encompassing the earliest organic structures which appeared between and billion years ago. The Paleozoic Era begins with the Cambrian Period ( million years ago) and includes the Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian Periods. This is followed by the Mesozoic Era, consisting of the Triassic, Jurassic, and Cretaceous Periods, ending approximately million years ago. The Cenozoic Era is Divided into the Tertiary Period—containing the Paleocene, Eocene, Oligocene, Miocene, and Pliocene Epochs—and the Quaternary Period, which includes the Pleistocene and the current Holocene Epoch.
Neoproterozoic Environmental Context and Glaciations
The Neoproterozoic era was marked by significant climatic shifts, most notably two major global glaciations identified through geological evidence such as tillites and cap carbonates. The Sturtian Glaciation was the longer of the two, lasting approximately million years from to Ma. The Marinoan Glaciation followed later, lasting approximately million years between and Ma. Isotopic records during this time show dramatic shifts in ͅ^{13}C\%, with values ranging between and . These glaciations represent the environmental backdrop against which the earliest complex animal fossils began to appear in the rock record.
Tracing the Origins of Complex Animal Life
Determining the timing of the evolution of complex life involves two primary lines of evidence: molecular clocks and the fossil record. Molecular clocks use groups with well-documented fossil records to calibrate the rate of molecular divergence, allowing scientists to measure the genetic difference between extant groups and estimate when they split from a common ancestor. This often suggests deeper origins for animal lineages than the fossil record explicitly shows. The fossil record provides physical evidence through body fossils and ichnofossils (trace fossils), which are markings in sediment made by animal activity. Neoproterozoic origins around Ma include single-celled animal-like organisms, specifically skeletonized amoebae. It is noted that amoebae are more closely related to animals and fungi than they are to plants.
The Search for the Earliest Sponges
There is significant debate regarding the earliest evidence of Porifera (sponges). One candidate is Otavia, a fossil from Namibia dated between Ma and Ma. More controversially, Elizabeth C. Turner published a study in Nature () titled "Possible poriferan body fossils in early Neoproterozoic microbial reefs," which describes potential sponge fossils from the Northwest Territories of Canada dated to Ma. However, the consensus view among paleontologists remains skeptical, as almost all claims for Precambrian sponges are strongly contested, with many experts suggesting these structures are not fossils at all. More definitive members of Porifera, such as Heliocolocellus cantori described by Wang et al. in , are dated more recently to between and Ma and are considered potential members of the crown group.
The Ediacara Biota and Exceptional Preservation
The "Ediacara biota" refers to the early metazoans that proliferated during the last million years of the Neoproterozoic. This fauna is named after the Ediacara Hills in Australia, where they were discovered in a lagerst{ a}tten. These organisms first appeared around Ma, with a major radiation occurring at Ma. Because these organisms were mostly soft-bodied, they required exceptional circumstances to fossilize. Such sites are known as Konservat-Lagerst{ a}tten. At Mistaken Point in Newfoundland, Ediacaran fossils were preserved by an ash layer that caused rapid burial in anoxic conditions. While these organisms lacked hard parts, they have been documented on every continent except Antarctica.
Morphology and Classification of Ediacaran Organisms
The biological relationships of the Ediacara fauna are highly debated; some see them as an evolutionary dead end (Vendianbionta), while others see them as early members of modern groups like Cnidaria or Annelida. Currently, many are grouped under Petalonamae. Specific examples include Charnia (a rangeomorph petalonamid), Pteridinium (an erniettomorpha petalonamid), and Arborea (an arboreomorph petalonamid). Spriggina is a notable fossil characterized by glide reflection rather than true bilateral symmetry, though it is considered a close relative or ancestor to bilaterally symmetrical organisms. While modern echinoderms exhibit five-fold symmetry, some Ediacaran forms exhibit three-fold or five-fold symmetry that may not be direct precursors to modern groups.
Advanced Ediacaran Forms and Modern Ancestry
Kimberella is a significant Ediacaran fossil initially classified as a jellyfish but now believed to be a triploblastic organism possessing three tissue layers, unlike cnidarians which have two. Though no radula has been identified in the fossils, Kimberella is found alongside feeding traces typically made by a radula, suggesting it may be a mollusk. Another definitive connection to modern groups is Auroralumina attenboroughii, which displays feeding tentacles. This trait has led to its assignment as a "stem" Medusozoan, a group that includes modern jellyfish, hydroids, and box jellyfish. These findings suggest that while many Ediacaran forms are mysterious, some are clearly linked to extant phyla.
Trace Fossils and the Small Shelly Fauna
Evidence of animal behavior is found in trace fossils. In very old siltstones, such as those from billion years ago with layers approximately thick, there is no evidence of burrowing. The first simplistic burrows do not appear until shortly before Ma. As the Neoproterozoic drew to a close, the fossil record began to show "Small Shelly Fauna" (SSF). A prime example is Cloudina, which produced millimeter-scale conical fossils made of calcareous cones around Ma. Though their exact identity is unknown—potentially polychaetes or bilateral organisms—these Small Shelly Fauna represent the beginning of the proliferation of skeletonized life that characterizes the start of the Cambrian Period.