The Handy Dinosaur Answer Book. Patricia Barnes-Svarney
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as eukaryotes, began to develop approximately 1.5 to 1.9 billion years ago, according to the known fossil record. Before this time, rock layers contained only tiny prokaryotes, such as bacteria and blue-green algae.
When did the first multicellular forms of life develop?
Based on the known fossil record, the first true primitive forms of multicellular life apparently developed around 650 million years ago, although some scientists classify a certain 1.2 billion-year-old red algae as a “taxonomically resolved” multicellular organism. (Humans are considered multicellular organisms, complete with 100 trillion cells that make up our bodies.)
One of the first such organisms is thought to have been a primitive form of sponge. The first fossil records of burrows are also found around the same time. These multicellular organisms are called Ediacaran fauna or assemblages (they are named after the Ediacaran hills in Southern Australia). Most have large surface areas, perhaps in response to their need to absorb oxygen, as there were very small concentrations of this gas present in the atmosphere at that time. They appear to have lived in shallow marine environments.
Did life develop more than just one time?
Many scientists believe that life may have started over and over on Earth. They speculate that once life began—either around ocean vents and/or in the shallow seas—comets and asteroids would strike the planet, killing off all the beginning stages of life. This may have happened many times over millions of years, until life became stable enough to sustain and diversify itself.
When did the first true plants appear on land?
Fossils called Cooksonia, found in Ireland, were probably the first true macroscopic plants to colonize land about 425 million years ago. Other plants also appeared not long after, including flowerless mosses, horsetails, and ferns. They reproduced by throwing out spores or minute organisms that carried the genetic blueprint for the plant. The ferns eventually developed seeds, but this did not happen until about 345 million years ago. Vascular plants—those with roots, stems, and leaves—evolved about 408 million years ago.
This yellow tube sponge, found near the Cayman Islands, descends from sponges that were among the first multicellular life on the planet (iStock).
When did the first soft-bodied animals appear in the oceans?
Fossils reveal that the first soft-bodied animals appeared about 600 million years ago in the oceans. They included a form of jellyfish, as well as segmented worms.
What is the oldest-known life form that existed on land?
So far, there is no definitive agreement about the oldest-known land life to have emerged on Earth, but there have been some intriguing discoveries. For example, in 1994 scientists in Arizona discovered fossilized tubular microorganisms dating back 1.2 billion years. In 2000, another team of scientists at NASA’s Astrobiology Institute uncovered an even older possibility: fossilized remnants of microbial mats (composed primarily of cyanobacteria) that developed on land between 2.7 billion and 2.6 billion years ago in the eastern Transvaal district of South Africa. Around 2002, yet another scientist uncovered what he thought may have been the earliest life on land in the form of a biocrust—a thin film of bacteria that covered stretches of sand in Scotland’s Torridon region. It is thought that the ripples in certain rocks actually represent billion-year-old biosignatures left behind by the first organisms to inhabit the land.
What were the first land animals and why did they move onto the dry land?
The first larger land animals to wander onto land were probably arthropods, such as scorpions and spiders. Many of these creatures have been found in Silurian period rock layers, usually in association with fossils of the oldest-known vascular land plants.
No one truly knows why the first animals moved from the oceans to dry land, but there are plenty of theories. One is that animals wanted to expand their territory, similar to the way many modern animals behave. Another possibility is that as more animals evolved, there would have been a higher demand for a better food source. By adapting to land life—and the “new” food sources on land—these organisms would have a better chance of survival.
When did the first primitive dinosaurs appear?
The first primitive dinosaurs appeared about 230 million years ago. They were much smaller, and less fierce, than the Tyrannosaurus rex we often think of when someone mentions the word “dinosaur.”
How long did it take for dinosaurs to evolve from the first land animals?
The first larger land animals that would eventually lead to the appearance of dinosaurs evolved around 440 million years ago. Dinosaurs then evolved around 250 million years ago. Thus, it took about 190 million years for dinosaurs to appear after the first land animals. Remember, these numbers are based on the currently known fossil record, and could change if new fossils are found.
GEOLOGIC TIME
What is geologic time?
Geologic time is the immense span of time that has elapsed since Earth first formed—almost 4.5 billion years ago—to recent times.
What is the geologic time scale?
The geologic time scale is a way of putting Earth’s vast history into an orderly fashion, giving a better perspective of events. At the turn of the nineteenth century, William Smith (1769–1839), an English canal engineer, observed that certain types of rocks, along with certain groups of fossils, always occurred in a predictable order in relation to each other. In 1815, he published a map of England and Wales geology, establishing a practical system of stratigraphy, or the study of geologic history layer-by-layer. Simply put, Smith proposed that the lowest rocks in a cliff or quarry are the oldest, while the highest are the youngest.
By observing fossils and rock type in the various layers, it was possible to correlate the rocks at one location with those at other locations. Smith’s work, combined with the first discoveries of dinosaur fossils in the early 1800s, led to a framework that scientists still use today to divide Earth’s long history into the geologic time scale, with its various, arbitrary divisions of time including eras, periods, and epochs. Established between 1820 and 1870, the time divisions are a relative means of dating; that is, rocks and fossils are dated relative to each other as to which are older and younger. It was not until radiometric dating was invented in the 1920s that absolute dates were applied to rocks and fossils—and to the geologic time scale.
What are the divisions of the geologic time scale?
The geologic time scale divisions have changed significantly over time, mainly because of new fossil discoveries and better radiometric dating techniques—and it will no doubt continue to change. The following table is a general listing of the geologic time table based on current interpretations of rocks and fossils.
How are the divisions on the geologic time scale named?
Most of the major divisions on the geologic time scale are based on Latin names, or areas in which the rocks were first found. For example, the Carboniferous period gets its name from the Latin words for “carbon-bearing,” in reference to the coal-rich rocks found in England; the Jurassic period is named after the Jura Mountains along the border of France and