The first eukaryotic life forms may be as old as 2 billion years ago. Multicellular algae were common by about 1.2 b.y. ago; wormlike creatures (possibly animals) had appeared by then. Best known among the Eucaryotic algae is Grypania spiralis, found in ancient rocks in Michigan and in Australia. This fossil is preserved because it formed simple shells:
Life continued in this primitive state until about a billion and a half years ago when photosynthesis became a common process that helped plant life flourish and released Oxygen. The first multicelled plant is similar to modern brown algae; pieces of this have been found in Chinese rocks dating to about 1.88 billion years ago.
In the Proterozoic (2.5 billion years to 540 million years), life forms slowly changed, with eukaryotic (nucleus) single-celled phylla becoming more diverse. Protozoa (primitive forms) were joined by Metazoa (more advanced) about 1 billion years ago. The first precursors to animals were ancestors to the sponges. Here is an example:
The oldest multicellular animal yet discovered is a sponge(like) fossil from rocks in Oman. These date at approximately 710 million years. No actual fossil form was recovered (sponges only have soft parts) but instead a distinctive chemical was found in the carbonate rocks. The telltale sign present in the rocks is a fatty chemical called 24-isopropylcholestane, which scientists have found only in the skeletal structures of demosponges, the most common member of the sponge family
Claims of a unicellular fossil trace in Australian rocks that are about 1.2 billion years old are still in dispute as the evidence could be the result of inorganic activity. Thiomargarita is the name given to markings in Chinese rocks about 600 million years old that have been interpreted as eggs or embryos, and by others as bacteria
The greatest explosion of life in earth history took place about 600 to 500 million years (late Proterozoic into Cambrian) ago with the appearance of distinctive and diverse animal forms. One significant marker was the first appearance of animals with bilateral symmentry at least 600 million years ago. These animals were very small, generally microscopic, and usually found in shales; the samples had to be thin-sectioned by guesswork to find the tiny objects which were preserved soft parts. Representative of these is Vernanimalcula, found in China:
Some taxonomists place the appearance of bilateralism even earlier - one school maintains about a billion years ago. The common ancestor to all animals has been named Unilateria (no such fossils have been found). A general scheme for the evolutionary chain of animals follows this lineage:
A significant upswing in the numbers and diversity of life marks the last 200 million years of the Proterozoic (often referred to as the "Neoproterozoic"). This is a time-stratigraphic chart that introduces Periods that will now be discussed:
Two of the most common guide fossils to Vendian life are shown here as actual specimens.
By the opening of the Cambrian, many forms of invertebrate life had developed (mainly for protection) external carapaces or coverings that, after death, survive as fossil shells. A survey of Cambrian stratigraphy indicates a dramatic increase in diverse marine life forms, so much so that this abundance of living creatures has been referred to as "The Cambrian Explosion".
Around 535 m.y. ago, in the early Cambrian, a soft parts assemblage of fossil forms has been found in the Chengjiang Formation in Yunnan Province of southern China. Here are typical fossils;
This is an artist's conception of typical animal and plant life in the shallow sea in which the Burgess shale was deposited:
Some of the typical life forms in the Burgess shale, each fascinating in its own right, are shown below:
An excellent review of early life on Earth is available at this web site maintained by the University of Munster (unfortunately, many of the links no longer are active).A second peak time in the abundance of shell-surviving life forms was in the Upper Ordovician (by this time also, the first larger vertebrates, fossil fish, had appeared). Below are two illustrations: the first, an artist' conception of marine invertebrate life in the late Ordovician; the second, a typical slab of Ordovician limestone (from Indiana) containing the fossil types listed in its caption:
The oldest known vertebrate life may be a tiny fish (8 cm in length) called Anatoleptis, of very late Cambrian age (510 million years; younger specimens (470 m/y.) have been found in Scandanavian rocks. The photo below is a microscope view of scales from this fish, whose remains have been found in Wyoming and other parts of North America:
Almost nothing is known about life dwelling mainly on land. A small millipede found in rocks in Scotland, dated at 428 million years ago, is a candidate for earliest terrestrial life:
There have been several extended time spans (but occuring mainly within single Periods) when major spurts in evolution have witnessed appearances of whole new classes of animal life forms. The Cambrian Explosion described above is the prime example. In the Devonian, the first amphibians (land-dwelling animals), insects, and land trees have been found in the fossil record. A "missing link" (375 million years old) between fish and amphibians was found on Ellesmere Island in the Canadian Arctic. Known as Tiktaalik roseae, it has a crocodilelike head, was 3 meters long, had fins with articulation that foretold jointed legs, and probably could wiggle like a seal on land when not in the sea. Here it is:
We have alluded to the huge increase in animal life since the "Cambrian Explosion" witnessed in the Burgess Shale. This is a good place in the narrative to summarize animal life since the end of the Precambrian.
Although the most ancient life was a mix of single celled plants and animals, we have not touched upon the more advanced forms of life since the Precambrian. A major and vital evolutionary spurt was when marine plant life adapted to living on the land. This began about 480 million years ago. These next two diagrams show the history of land plants:
The illustration below show modern brown algae which were likely progenitors of the land plants.
By the Devonian land plants, which had started as ferns, club mosses, horsetails, and liverworts (all of these still exist), had established vascular systems, acquired the ability to root in the soil, and were able to propagate using seeds. One of the first giant trees is Archaeopteris, shown in this drawing:
A typical plant in this early stage of evolution is Cooksonia, shown here as a fossil frong:
The best known locality for early land plant fossils is the Rhynie chert deposits of Scotland:
This drawing below is a reconstruction of a typical Devonian forest:
The Carboniferous forests are among the most diverse in geologic history. Their decay in swamps has produced coal beds world wide.
Among the giant plants in the Carboniferous forest shown in this panel were Cordaites, an early relative of conifers; Calamites, a bushy horsetail; Medullosa,a seed fern (a plant with seeds and fern-like leaves); Psaronius, a tree fern; and Paralycopodites and Lepidophloios, lycopsids (scaly, pole-like trees with cones). Lepidophloios could grow to 40 m (132 ft), but most of today�s lycopsids, known as quillworts and club mosses, grow only a few centimeters high.
The illustration below shows a cluster of leaves of Pennsylvanian age:
As mentioned in Section 18 and elsewhere, some of the major advances or disappearances in life forms are being attributed to worldwide effects of large impacts. But other causes of these compressed spurts or declines are probably involved in other cases: extreme volcanism; dramatic climate changes; continental splitting, are suggested. These may be manifestations of the "Punctuated Equilibrium" mode of evolutions proposed by Stephen Jay Gould and others.
A rather fanciful panorama of life forms (Kingdoms, Phylla, Families, etc.) from the Late Precambrian to the Present is shown in this mural that is found on the Humboldt State University campus. On the left, pre-Paleozoic animals with soft and hard parts give way to the Invertebrates of the Early Paleozoic, the first fish (Ordovician; sharks in the Devonian), then Amphibians that appeared at about the same time (Mississipian) that land plants took root, with the first reptiles and dinosaurs near the end of the Paleozoic (about the time the first extensive forests spread in the Pennsylvanian), reptiles and small mammals in the Mesozoic, along with the first birds, and finally a dominance of mammals, flowering plants, and widespread forests in the Cenozoic.
Missing from the far right of this panel is the story of the hominids, which includes today's mankind. Ancestors to the hominids have been found as far back as 4.5 million years (older animals that may be links are still controversial). This subject is far too involved for any extended treatment in this Tutorial, but the following synopsis touches upon many of the key ideas. The study of ancient precursors to modern humans, is called Paleoanthropology; see this Wikipedia site for a review of mankind's genealogy.
A good, quick overview that carries back to the beginnings of life 4 billion years ago but gives some emphasis to the appearance of humans is found in the Wikipedia Timeline of Human Evolution webpage. Before embarking on the remainder of this page, you should benefit from looking at the list of hominids and their precursors at this Wikipedia list of hominids. These three diagrams are also helpful. The first is a chart that gives a general time line through the Cenozoic and into the end of the Mesozoic; the second shows one version of primate evolution; the third a variation of this with hominids included and broadly spelled out:
Classification of the Primates show two broad groupings: 1. The Prosimians, made up of Lemurs, and 2. The Anthropoids, that includes Old and New World monkeys, the Apes, the Chipanzees, and Humans. Of the Anthropoids, there are two major divergences over the last 30 million years. One line includes Old World monkeys, the other contains several branches of which one is the hominids. While the closest tie between Man and other similar animals is said by many to be with the Gorilla or other Apes, the Chimpanzees are also related (98% of the thousands of DNA genes in the genomes of Man and Chimp are identical and located in the same relative positions).
Much publicity has been given to an announcement in May of 2009 of what is called by some as the "missing link" ancestor of both Primate Groups. A nearly complete primate fossil was found in the 1980s in the Messel shale quarry near Frankfurt, Germany. The rocks there are 47 million years in age, placing the many fossil types at Messel in the Eocene.The primate's formal name is Darwinius messelae, but it is called "Ida" (after the daughter of the scientist who led the recent research on it). These two illustrations are pertinent:
The fossil appears to be a transitional link between the Prosimians and the Anthropoids. It has some lemurlike qualities but lacks certain features of that group while showing some bones that link it to the Anthropoids. Of particular import is the opposable thumb in the hand; this is what is characteristic of primates among mammals and a trait that facilitated the ability of humans to evolve into civilized Man.
We will now concentrate on human evolution, as determined mainly from the fossil record. As a generality, the genus "Homo" is associated with modern humans and direct ancestors whereas the genus "Australopithecus" and others refer to pre-humans. Two diagrams are introduced here. The first emphasizes the time spans during which the various genera lived; the second shows one model (there are competing versions) of the lineage, as a family tree, of the anthropoids back to about 7 million years.
Part of this progression can be displayed in terms of an evolutionary pattern that depends on the principal evidence - interpretation of skeletal remains, in this case reconstructed heads.
The diagram below shows the skull differences in a different rendition:
This next diagram was taken off the Internet; it is a reproduction of an August 1999 feature article in Time Magazine. It is worth examining before we add comments:
Fossil bones of various hominids have been found on all Old World continents, as evident in this map:
This next map incorporates the above map but shows the general pathway to the New World (N. and S. America). The blue lines are one of several proposed pathways from central Africa to the other continents (discussed below):