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7.2.2A: Pre-Cambrian Animal Life - Biology

7.2.2A: Pre-Cambrian Animal Life - Biology


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Early animal life (Ediacaran biota) evolved from protists during the pre-Cambrian period, which is also known as the Ediacaran period.

Learning Objectives

  • Describe the types of animals found in the Ediacaran period

Key Points

  • The pre-Cambrian period ( Ediacaran period ) took place between 635-543 million years ago.
  • Early animal life, called Ediacaran biota, evolved from protists; it was previously believed early animal life included only tiny, sessile, soft-bodied sea creatures, but scientific evidence suggests more complex animals lived during this time.
  • Sponge-like fossils believed to represent the oldest animals with hard body parts, named Coronacollina acula, date back as far as 560 million years.
  • The fossils of the earliest animal species ever found were small, one-centimeter long, sponge-like creatures, dating before 650 million years, which predates the Ediacaran period.
  • The discovery of the fossils of the earliest animal species provided evidence that animals may have evolved before the Ediacaran period during the Cryogenian period.

Key Terms

  • Ediacaran period: period from about 635-543 million years ago; the final period of the late Proterozoic Neoproterozoic Era
  • choanoflagellate: any of a group of flagellate protozoa thought to be the closest unicellular ancestors of animals
  • Coronacollina acula: sponge-like fossils believed to represent the oldest animals with hard body parts that date back as far as 560 million years

Pre-Cambrian Animal Life

The time before the Cambrian period is known as the Ediacaran period (between 635-543 million years ago), the final period of the late Proterozoic Neoproterozoic Era. It is believed that early animal life, termed Ediacaran biota, evolved from protists at this time. Some protist species called choanoflagellates closely resemble the choanocyte cells in the simplest animals, sponges. In addition to their morphological similarity, molecular analyses have revealed similar sequence homologies in their DNA.

The earliest life comprising Ediacaran biota was long believed to include only tiny, sessile, soft-bodied sea creatures. However, recently there has been increasing scientific evidence suggesting that more varied and complex animal species lived during this time, and possibly even before the Ediacaran period.

Fossils believed to represent the oldest animals with hard body parts were recently discovered in South Australia. These sponge-like fossils, named Coronacollina acula, date back as far as 560 million years. They are believed to show the existence of hard body parts and spicules that extended 20–40 cm from the main body (estimated about 5 cm long). Other organisms, such as Cyclomedusa and Dickinsonia, also evolved during the Ediacaran period.

Another recent fossil discovery may represent the earliest animal species ever found. While the validity of this claim is still under investigation, these primitive fossils appear to be small, one-centimeter long, sponge-like creatures. These fossils from South Australia date back 650 million years, actually placing the putative animal before the great ice age extinction event that marked the transition between the Cryogenian period and the Ediacaran period. Until this discovery, most scientists believed that there was no animal life prior to the Ediacaran period. Many scientists now believe that animals may, in fact, have evolved during the Cryogenian period.


Precambrian

The Precambrian (4500 to 543 million years ago) is a vast period of time, nearly 4,000 million years long, that began with the formation of the Earth and culminated with the Cambrian Explosion. The Precambrian accounts for seven-eighths of our planet's history.

Numerous important milestones in the development of our planet and the evolution of life occurred during the Precambrian. The first life arose during the Precambrian. The tectonic plates formed and began shifting across the surface of the Earth. Eukaryotic cells evolved and the oxygen these eary organisms exhaled collected in the atmosphere. The Precambrian drew to a close just as the first multicellular organisms evolved.

For the most part, considering the immense length of time encompassed by the Precambrian, the fossil record is sparse for that time period. The oldest evidence of life is encased in rocks from islands off of western Greenland. Theses fossils are 3.8 billion years old. Bacteria that is more than 3.46 billion years old was discovered in Western Australia. Stromatolite fossils have been discovered that date back 2,700 million years.

The most detailed fossils from the Precambrian are known as the Ediacara biota, an assortment of tubular and frond-shaped creatures that lived between 635 and 543 million years ago. The Ediacara fossils represent the earliest known evidence of multicellular life and most of these ancient organisms appear to have vanished at the end of the Precambrian.

Although the term Precambrian is somewhat outdated, it is still widely used. Modern terminology disposes of the term Precambrian and instead divides the time before the Cambrian Period into three units, the Hadean (4,500 - 3,800 million years ago), the Archean (3,800 - 2,500 million years ago), and the Proterozoic (2,500 - 543 million years ago).


More Complexity Appears

The first traces of eukaryotic cells showed up about 2.1 billion years ago according to the fossil record. These seem to be single-celled eukaryotic organisms that lacked the complexity we see in most of today's eukaryotes. It took about another billion years before the more complex eukaryotes evolved, probably through endosymbiosis of prokaryotic organisms.

The more complex eukaryotic organisms began living in colonies and creating stromatolites. From these colonial structures most likely came multicellular eukaryotic organisms. The first sexually reproducing organism evolved around 1.2 billion years ago.


Pre-Cambrian Animal Life

The time before the Cambrian period is known as the Ediacaran period (from about 635 million years ago to 543 million years ago), the final period of the late Proterozoic Neoproterozoic Era (Figure). It is believed that early animal life, termed Ediacaran biota, evolved from protists at this time. Some protist species called choanoflagellates closely resemble the choanocyte cells in the simplest animals, sponges. In addition to their morphological similarity, molecular analyses have revealed similar sequence homologies in their DNA.

(a) Earth’s history is divided into eons, eras, and periods. Note that the Ediacaran period starts in the Proterozoic eon and ends in the Cambrian period of the Phanerozoic eon. (b) Stages on the geological time scale are represented as a spiral. (credit: modification of work by USGS)

The earliest life comprising Ediacaran biota was long believed to include only tiny, sessile, soft-bodied sea creatures. However, recently there has been increasing scientific evidence suggesting that more varied and complex animal species lived during this time, and possibly even before the Ediacaran period.

Fossils believed to represent the oldest animals with hard body parts were recently discovered in South Australia. These sponge-like fossils, named Coronacollina acula, date back as far as 560 million years, and are believed to show the existence of hard body parts and spicules that extended 20–40 cm from the main body (estimated about 5 cm long). Other fossils from the Ediacaran period are shown in Figureab.

Fossils of (a) Cyclomedusa and (b) Dickinsonia date to 650 million years ago, during the Ediacaran period. (credit: modification of work by “Smith609”/Wikimedia Commons)

Another recent fossil discovery may represent the earliest animal species ever found. While the validity of this claim is still under investigation, these primitive fossils appear to be small, one-centimeter long, sponge-like creatures. These fossils from South Australia date back 650 million years, actually placing the putative animal before the great ice age extinction event that marked the transition between the Cryogenian period and the Ediacaran period. Until this discovery, most scientists believed that there was no animal life prior to the Ediacaran period. Many scientists now believe that animals may in fact have evolved during the Cryogenian period.


(D) Samuel A. Bowring et al., “Calibrating Rates of Early Cambrian Evolution,” Science 261 (1993): 1293-1298.

What significance does the Cambrian explosion have for evaluating Darwin’s theory that all animals are modified descendants of a common ancestor? As we have seen, Darwin himself considered it a serious problem (Excerpt A). Although Darwin’s theory predicts that animal evolution should proceed from the “bottom up,” with the largest differences emerging last, James Valentine and his colleagues wrote in 1991 that the pattern of the Cambrian explosion “creates the impression that metazoan evolution has by and large proceeded from the ‘top down’ ” (Excerpt B, p. 294). Harry Whittington, an expert on the Cambrian fossils from the Burgess shale, wrote in 1985: “It may well be that metazoan animals arose independently in different areas. I look sceptically upon diagrams that show the branching diversity of animal life through time, and come down at the base to a single kind of animal” (Excerpt F, p. 131). Evolutionary biologist Jeffrey Levinton, though convinced of the common ancestry of animals, acknowledged in 1992 that the Cambrian explosion — “life’s big bang,” as he called it — remains “evolutionary biology’s deepest paradox” (Excerpt G, p. 84). Although “the body plans that evolved in the Cambrian by and large served as the blueprints for those seen today,” Levinton saw “no reason to think that the rate of evolution was ever slower or faster than it is now. Yet that conclusion still leaves unanswered the paradox posed by the Cambrian explosion and the mysterious persistence of those ancient body plans” (Excerpt G, pp. 84, 90). In 1999, University of California biologist Malcolm Gordon wrote: “Recent research results make it seem improbable that there could have been single basal forms for many of the highest categories of evolutionary differentiation (kingdoms, phyla, classes)” (Excerpt H, p. 331). Gordon concluded: “The traditional version of the theory of common descent apparently does not apply to kingdoms [i.e., plants, animals, fungi, bacteria] as presently recognized. It probably does not apply to many, if not all, phyla, and possibly also not to many classes within the phyla” (Excerpt H, p. 335).


Vanished Precambrian Life Forms Were Like Nothing Before or Since


For a brief span of time, about 542 million years ago, the world belonged to the Ediacarans, a group of life forms so physiologically unique that biologists have considered giving them their own taxonomic kingdom.

An essay by Richard Corfield in Astrobiology Magazine points up the strange history of the Ediacarans, a group of anatomically diverse organisms that lived during the Ediacaran period (between 635 and 542 million years ago). These creatures, which predated nearly every form of animal life that exists today, stood rooted in bacterial bases on the seafloor, drawing nutrients from the water.

As best we can tell, the Ediacarans lacked mouths and recognizable digestive systems, and their bodies are thought to have looked like "sacks of mud, disks, hubcaps and mattresses." They were among the first complex life forms to appear on the planet, but they bear no discernible resemblance to anything else in the fossil record:

. they have none of the characteristics of the bilaterian animals, which evolved during the Cambrian explosion 542 million years ago. Since then, bilateral animals have provided the basic body plan for every animal that has occupied and dominated the Earth.

The Ediacarans came and went in a remarkably brief interval, geologically speaking. Corfield believes their population exploded when a mass-oxygenation of Earth's oceans coincided with the end of the Cryogenian ice age. Before long, Ediacarans had spread across the globe, as indicated by fossils everywhere from England to Australia. But the rise of mobile, bilaterian animals introduced too much competition to the oceans, and the Ediacarans died off almost as quickly as theyɽ flourished.

Guy Narbonne, a paleontologist at Queen's University in Ontario, recently told reporters that Ediacaran-descended life could still be with us today, in the form of certain worms and mollusks. For Corfield, though, the interesting thing about Ediacarans isn't whether they still have a presence on Earth, but what their evolutionary arc suggests about whether and how life might develop on other worlds:

the story of the Ediacarans. tells us that evolution can happen very quickly. The idea — first credited to Darwin — that vast amounts of deep time are required for evolution to occur may not be correct. The speed with which the Ediacarans arose in the aftermath of the final Cryogenian glaciation suggests strongly that the evolution of complex, multicellular organisms was on the blocks and just waiting for the starting pistol.

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DISCUSSION

A well written and accessible book on this subject that discusses the several Cambrian phylums and taxa discovered in the Burgess shale is Wonderful Life by Stephen Jay Gould (1989).

Gould was a professor at Harvard and a gifted as well as prolific writer who in addition to writing numerous books also wrote a monthly column for Natural History. I always felt he modeled himself along the lines of Carl Sagan, sharing similar roots, as someone who wanted to popularize scientific subjects, in this case evolutionary biology.

He uses the Burgess fossils to reinforce his own theory of punctuated equilibrium (with Niles Eldridge) and of the contingency of history as well as to discount Simon Morris' convergent evolution hypothesis. An interesting read, particularly when compared to the more dogmatic Richard Dawkins. The two scholars of evolution had quite the entertaining feud with Dawkins a proponent of the gradualism who frequently, and harshly, rebuked Gould for his views.

It was books like these that encouraged me to abandon reading science fiction in favor of actual science (among other subjects) many decades ago. It turns out reality is far more fascinating and amazing then anything that can be imagined.


The Cambrian Explosion of Animal Life

If the fossils of the Ediacaran and Cryogenian periods are enigmatic, those of the following Cambrian period are far less so, and include body forms similar to those living today. The Cambrian period, occurring between approximately 542–488 million years ago, marks the most rapid evolution of new animal phyla and animal diversity in Earth’s history. The rapid diversification of animals that appeared during this period, including most of the animal phyla in existence today, is often referred to as the Cambrian explosion. Animals resembling echinoderms, mollusks, worms, arthropods, and chordates arose during this period. What may have been a top predator of this period was an arthropod-like creature named Anomalocaris, over a meter long, with compound eyes and spiky tentacles. Obviously, all these Cambrian animals already exhibited complex structures, so their ancestors must have existed much earlier.

One of the most dominant species during the Cambrian period was the trilobite, an arthropod that was among the first animals to exhibit a sense of vision. Trilobites were somewhat similar to modern horseshoe crabs. Thousands of different species have been identified in fossil sediments of the Cambrian period not a single species survives today.

The cause of the Cambrian explosion is still debated, and in fact, it may be that a number of interacting causes ushered in this incredible explosion of animal diversity. For this reason, there are a number of hypotheses that attempt to answer this question. Environmental changes may have created a more suitable environment for animal life. Examples of these changes include rising atmospheric oxygen levels and large increases in oceanic calcium concentrations that preceded the Cambrian period. Some scientists believe that an expansive, continental shelf with numerous shallow lagoons or pools provided the necessary living space for larger numbers of different types of animals to coexist. There is also support for hypotheses that argue that ecological relationships between species, such as changes in the food web, competition for food and space, and predator-prey relationships, were primed to promote a sudden massive coevolution of species. Yet other hypotheses claim genetic and developmental reasons for the Cambrian explosion. The morphological flexibility and complexity of animal development afforded by the evolution of Hox control genes may have provided the necessary opportunities for increases in possible animal morphologies at the time of the Cambrian period. Hypotheses that attempt to explain why the Cambrian explosion happened must be able to provide valid reasons for the massive animal diversification, as well as explain why it happened when it did. There is evidence that both supports and refutes each of the hypotheses described above, and the answer may very well be a combination of these and other theories.

However, unresolved questions about the animal diversification that took place during the Cambrian period remain. For example, we do not understand how the evolution of so many species occurred in such a short period of time. Was there really an “explosion” of life at this particular time? Some scientists question the validity of this idea, because there is increasing evidence to suggest that more animal life existed prior to the Cambrian period and that other similar species’ so-called explosions (or radiations) occurred later in history as well. Furthermore, the vast diversification of animal species that appears to have begun during the Cambrian period continued well into the following Ordovician period. Despite some of these arguments, most scientists agree that the Cambrian period marked a time of impressively rapid animal evolution and diversification of body forms that is unmatched for any other time period.


But wait, glaciers grind down mountains and dump the tailings in the ocean–a perfect mining operation. And I thought ice ages were a waste of time.

In the comments of this UD post from yesterday, (comment #21) I referred to Nick Matzke’s rant over at the Panda’s Thumb yesterday as yet another illustration of the double standard’s Matzke’s has when it comes to his critiques of anyone who dares challenge Darwinian Orthodoxy. In my comments yesterday, I gave an example of Read More…


Pre-Cambrian

Hadean: the Earth we live on is approximately ( ext<4.6>) billion years old. The conditions that allowed for the emergence of life lasted approximately ( ext<500>) million years. It was an environment in which the Earth’s crust cooled and the oceans and atmosphere began to form. In this environment, a variety of complex chemical reactions occurred, resulting in the production of the earliest molecules capable of making copies of themselves.

Archean: when life in the form of uni-cellular organisms first developed, the Earth’s early atmosphere consisted entirely of volcanic gases, and there was no free oxygen. Prokaryotes evolved, with photosynthesising bacteria (known as cyanobacteria) emerging approximately 3 billion years ago. The presence of photosynthesising cyanobacteria resulted in the release of oxygen into the atmosphere.

The early single-celled organisms (bacteria and cyanobacteria) lived together in aquatic colonies. These colonies were formed by trapping sediments and minerals floating in water and by producing a mucus which bound everything together. As the colony of bacteria, minerals and sediments grew, so did the structure that they made, and layer upon layer built up. In order to capture as much sunlight as possible, the shape of the top of this sedimented algal mat was curved. These curved and multi-layered structures, called stromatolites, were preserved and exist as fossils. Some stromatolites survive to this day, in scattered locations around the globe (see figure below).

Stromatolites have been found at Wondergat, Northwest Province, South Africa.

Stromatolites in Shark Bay, Australia.

Proterozoic: over the next 800 million years, the earliest forms of sexual reproduction developed, thus greatly increasing the diversity of organisms. The first multicellular organisms consisting of cells specialised to perform specific functions began to evolve. The soft-bodied organisms, known as Swartpuntia, first appeared approximately ( ext<600>) million years ago and went extinct by the start of the Cambrian, ( ext<543>) million years ago. Swartpuntia fossils were discovered in Namibia and are some of the oldest fossils known to exist from before the Cambrian period.

Earliest soft-bodied animals found in Namibia. A simplified reconstruction of Swartpuntia.

Did You Know?

Approximately ( ext<850>) – ( ext<630>) million years ago, a ‘global glaciation’ event is thought to have occurred, also known as an ‘Snowball Earth’. At this time temperatures dropped dramatically and large parts of the Earth’s surface were covered in ice. During this period it is thought that any life forms dependent on light would have gone extinct.


A Survey of Precambrian Animals

Over the last few days, I’ve been doing some research into Precambrian animals. This is a direct effort to refute the claims of Meyer that all these forms appeared in the Cambrian explosion. This is the status of my efforts to date.

Age (mya) Reference Event Notes
3,500 Schopf 1993 microfossils resembling cyanobacteria (Australia)
2,100 Han & Runnegar 1992 Algal cysts with cells largers than any known in modern prokaryotes
1,500 Javaux et al 2001 complex specrical and spinose forms of eukaryotes
1,100 red and green algae appeared
1,000 Wray 1996 Time of divergence of inverts and verts molecular data A literal reading of the fossil record suggests that the animal phyla diverged in an ‘explosion’ near the beginning of the Cambrian period. Calibrated rates of molecular sequence divergence were used to test this hypothesis. Seven independent data sets suggest that invertebrates diverged from chordates about a billion years ago, about twice as long ago as the Cambrian. Protosomes apparently diverged from chordates well before echinoderms, which suggests a prolonged radiation of animal phyla. These conclusions apply specifically to divergence times among phyla the morphological features that characterize modern animal pbody plans, such as skeletons and coeloms, may have evolved later.
635-580 Budd, 2007 Doushanto Fm. The significance of these dates is that all of the Doushantuo fossils would predate the oldest of the famous Ediacaran fossils such as Dickinsonia, and thus would provide an independent record of the animal life during a period of time that no large body or trace fossils are known. Indeed, the overlying Dengying Fm does yield Ediacaran-type fossils, which could be said to support this contention.
630 Budd, 2007 Stem Group Animals Present
600 no multicellular prior to this
590 Xiao et al 1998, 2002 brown algae highly diverse multicellular algae
590 Xiao et al 1998, 2002 probably microscopic embryos in stages of cleavage with 2, 4, or 8 cells Ediacaran
590 Li et al 1998 sponges with monaxonal spicules and preserved soft tissue
590 Chen et al 2008 Bilaterian fossils appear Furthermore, the organization of these fossils, taken together with their provenance, indicates that the genetic tool kit and pattern formation mechanisms required for bilaterian development had already evolved by Doushantuo times, long before the Cambrian. Therefore, the diversification of body plans in the Early Cambrian followed from the varied deplument of these mechanisms once conditions permitted, not from their sudden appearance at od just before the Cambrian boundary.
575 Narbonne & Gehling 2002 Mackenzie Mountains, Canada: organisms in disc, trianle, and fronds
late Precam Fedonkin 1997 Kimbrella – a bilateral mollusc-like organism http://www.nature.com/nature/journal/v388/n6645/full/388868a0.html
541 Cambrian Period Begins
505 Caron 2006 Odontogriphus omalus was originally described as a problematic non-biomineralized lophophorate organism. Here we reinterpret Odontogriphus based on 189 new specimens including numerous exceptionally well-preserved individuals from the Burgess Shale collections of the Royal Ontario Museum. This additional material provides compelling evidence that the feeding apparatus in Odontogriphus is a radula of molluscan architecture comprising two primary bipartite tooth rows attached to a radular membrane and showing replacement by posterior addition. Further characters supporting molluscan affinity include a broad foot bordered by numerous ctenidia located in a mantle groove and a stiffened cuticular dorsum. Odontogriphus has a radula similar to Wiwaxia corrugata but lacks a scleritome. We interpret these animals to be members of an early stem-group mollusc lineage that likely originated in the Neoproterozoic Ediacaran Period, providing support for the retention of a biomat-based grazing community from the late Precambrian until at least the Middle Cambrian.

Notice that last entry “Cambrian period begins”. All of the stuff on this list is Pre-Cambrian. We have the presence of bilateral organisms. Indeed according to the Wray paper, we have the majority of the animal forms already present. That being said, specific features may have appeared later, but the genetic plan for those phyla are already present.

The Wray paper is particularly interesting as it also opens with a brief discussion of Darwin and how the appearance of the sudden development of animal fossils posed a problem for natural selection. The paper describes the use of molecular divergence rates for a large number of taxa. They used seven different genes for this process and the results are pretty interesting.

The existence of an extended but cryptic Precambrian history of metazoans also has some interesting implications for understanding the origin and diversification of animal body architecture. In particular, the rapid appearance of diverse skeletonized taxa in the fossil record during the middle Early Cambrian may reflect an exceptional period of simultaneous morphological innovation within distinct lineages rather than a rapid branching of phyla. It has long seemed likely, for example, that mineralized skeletons evolved independently in several phyla at this time (39). It is unlikely, however,that all “body plan” features evolved during the Cambrian. A cephalized, bilaterally symmetrical body composed of three germ layers predates the protostome-deuterostome split (28, 40, 41) and thus probably evolved much earlier than is generally recognized. Coeloms are shared by the two deuterostome phyla we examined and may predate the Cambrian by several hundred million years (the coeloms of protostomes may have an independent origin, and dating their appearance will require more
information about the divergence times of the various protostome phyla) .

Four of the major ‘innovations’ in body plans are in place well before the Cambrian. Radial anatomy, bilateral anatomy (with a specific head and rear), three germ-layers (we have three germ layers, it’s basically a difference in cell types that make up the outside layer, the inside tube (intestines, stomach , etc) and everything between those two layers (in humans things like the heart, skeleton, etc), and coeloms (a fluid-filled body cavity between the intestines and the body wall of some higher metazoans) were present long before the Cambrian.

We also have molluscs (Caron 2006) and cnidarians (Chen 2002) well before the Cambrian. We also have segmented organisms (Valentine 1989) and worms.

So, what’s Meyer’s issue here? Aside from the fact that there’s only four of these articles listed as references in his book (they are bolded below) and he draws completely opposite conclusions from them than the authors do.

Meyer’s book, even the Kindle edition, isn’t crosslinked from references back to text, so we’ll just have to see what he says when we find them.

A final note is that this isn’t even close to all the material on precambrian organisms. This is just a sample… i.e. what was available in PDF for free or via Science.

We’ll talk about Meyer’s “diagrams” in the next installment. Suffice to say that he must not have actually read the papers he referenced in his book, because his diagrams are nothing like what is actually in the papers.