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Authors: Peter Ward
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its ilk. Animals are less than a billion years old, and while the exact age of the first animal keeps getting put back into older-aged rocks, based on ever more sophisticated means of detecting their presence, there isstill no known fossil evidence of animals much older than that last snowball event. But in a way this is an argument over rather small chunks of time, compared to the vast interval that life has been on this planet. There are, of course, many types of multicellular organisms, including a considerable diversity of prokaryotic forms, and there is no doubt that the evolutionary invention of life with more than a single cell goes back to more than 2 billion years ago. But in most cases these multicellular prokaryotes are composed of only two cell types, and none would be mistaken for an animal.
Cellular slime molds are multicellular, as are some cyanobacteria and one group of magnetotactic bacteria. In a way, however, these are evolutionary dead ends (unless, of course, you are a slime mold; however, this group ultimately gave rise to little else but slime molds). They have existed on Earth for more than several billion years, and are highly conservative in an evolutionary sense. More complex are the multicellular plants that appeared more than a billion years ago, species probably looking very much like the green and red algae found on any seashore, from the intertidal zone down to the levels that light can penetrate. Animals, however, are younger yet.
The size of organisms seems to show some relationship to the appearance of oxygen in the atmosphere. Oxygen has allowed larger size than times prior to oxygen, and biological adaptations increasing the rate and/or volume of oxygen acquisition have often lead to gigantism. 4 The best example of this will be described in a later chapter, showing how the gigantism in dinosaurs was caused by a new kind of highly efficient lung and respiratory system design.
Fossils of true animals first appear in abundance about 600 million years ago. About this time the rock record shows the first evidence of “trace fossils,” the trackways or feeding records of ancient animals preserved not as body fossils in sediment, but as activity fossils—the record of ancient behavior. By that time oxygen levels were approaching (but not yet reaching) modern levels. Not only free oxygen but ozone levels had also reached relatively high concentrations, and thus much of the hard ultraviolet and other radiation reaching the Earth’s surface in earlier times was muted.
Geobiologist Andy Knoll of Harvard University on Neoproterozoic rocks exposed in East Greenland on an anomalously sunny day. (Copyright Andy Knoll, used with permission)
THE CURIOUS ORGANISMS KNOWN AS ACRITARCHS
In any discussion of Precambrian life, acritarchs actually make up a fair bit of the conversation. They appeared early on Earth: some of the oldest seem to appear around 3.2 billion years ago, and they then continue all the way into the time of animals. Yet the fact that they are a “garbage can” taxon, meaning that any number of not only different species but even different kingdoms and domains of organisms get placed in this catchall name, is just one more indication of how poorly we know the history of life before fossils became common in the time of animals and higher plants.
While being among the oldest-known multicellular fossils, first appearing at the almost unfathomable time period of 2 billion years ago, they remained relatively rare. But halfway through the Proterozoic era, or about a billion years ago, they started to increase in diversity,size, abundance, and morphological complexity in shape. The increase in complexity was generally marked by an increase in the number of spines extending from their small, spherical bodies. From 1 billion to 850 million years ago they remained common, and then the Cryogenian period began, with the enormous global changes that gave this time interval its name, the
Cellular slime molds are multicellular, as are some cyanobacteria and one group of magnetotactic bacteria. In a way, however, these are evolutionary dead ends (unless, of course, you are a slime mold; however, this group ultimately gave rise to little else but slime molds). They have existed on Earth for more than several billion years, and are highly conservative in an evolutionary sense. More complex are the multicellular plants that appeared more than a billion years ago, species probably looking very much like the green and red algae found on any seashore, from the intertidal zone down to the levels that light can penetrate. Animals, however, are younger yet.
The size of organisms seems to show some relationship to the appearance of oxygen in the atmosphere. Oxygen has allowed larger size than times prior to oxygen, and biological adaptations increasing the rate and/or volume of oxygen acquisition have often lead to gigantism. 4 The best example of this will be described in a later chapter, showing how the gigantism in dinosaurs was caused by a new kind of highly efficient lung and respiratory system design.
Fossils of true animals first appear in abundance about 600 million years ago. About this time the rock record shows the first evidence of “trace fossils,” the trackways or feeding records of ancient animals preserved not as body fossils in sediment, but as activity fossils—the record of ancient behavior. By that time oxygen levels were approaching (but not yet reaching) modern levels. Not only free oxygen but ozone levels had also reached relatively high concentrations, and thus much of the hard ultraviolet and other radiation reaching the Earth’s surface in earlier times was muted.
Geobiologist Andy Knoll of Harvard University on Neoproterozoic rocks exposed in East Greenland on an anomalously sunny day. (Copyright Andy Knoll, used with permission)
THE CURIOUS ORGANISMS KNOWN AS ACRITARCHS
In any discussion of Precambrian life, acritarchs actually make up a fair bit of the conversation. They appeared early on Earth: some of the oldest seem to appear around 3.2 billion years ago, and they then continue all the way into the time of animals. Yet the fact that they are a “garbage can” taxon, meaning that any number of not only different species but even different kingdoms and domains of organisms get placed in this catchall name, is just one more indication of how poorly we know the history of life before fossils became common in the time of animals and higher plants.
While being among the oldest-known multicellular fossils, first appearing at the almost unfathomable time period of 2 billion years ago, they remained relatively rare. But halfway through the Proterozoic era, or about a billion years ago, they started to increase in diversity,size, abundance, and morphological complexity in shape. The increase in complexity was generally marked by an increase in the number of spines extending from their small, spherical bodies. From 1 billion to 850 million years ago they remained common, and then the Cryogenian period began, with the enormous global changes that gave this time interval its name, the
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