The Handy Dinosaur Answer Book. Patricia Barnes-Svarney

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      One of the earliest forms of life to appear on Earth was cyanobacteria (inset), which have left behind unusual fossil rocks called stromatolites. (iStock).

      The second theory of life centers around a discovery made within the last half century: hydrothermal vents, which are cracks caused by volcanic magma seeping through the deep ocean floor. There were probably many more hydrothermal vents during the early history of Earth, as the crust was newer, and thus thinner, than today’s cooled, thicker crust. The organisms around these vents did not need to rely on photosynthesis for energy. Scientists know that today’s volcanic vent organisms live off the bacteria around the vents, which in turn extract energy from the hot, hydrogen sulfide-rich water found around the sunless cracks in the ocean floor. Early organisms could have survived in much the same way.

      In actuality, the conditions described by both theories could have existed simultaneously to produce the planet’s early life.

      What are the oldest-known fossils found in rock on Earth?

      The oldest-known fossils in rock have been found in Australia. One set of fossils found in Western Australia is dated between 3.45 and 3.55 billion years old. They show evidence of layered mounds of limestone sediment called stromatolites, which were formed by primitive microorganisms similar to blue-green algae called cyanobacteria. Scientists know that stromatolites exist today. The fossils look amazingly like the stromatolites from the shallow waters off the coast of modern Australia.

      There are other contenders for the oldest-known fossils. Tiny, simple cells have also been found in ancient cherts (crystalline-rich sedimentary rocks) from Australia, and there are similar ones in Africa. These cells are preserved by the silica from the chert, and appear to show a cell wall of some kind.

      When did the basic form of life develop on Earth?

      It is thought that as far back as about 3.8 billion years ago, a basic form of life was present on Earth. This life took the form of tiny cells, which were surrounded by membranes to isolate and protect their interiors from the surrounding environment. The cells had a basic genetic system similar to those in modern cells, and this allowed the cells to self-replicate. We classify these earliest life-forms as prokaryotes, which includes such organisms as bacteria and cyanobacteria.

      When did larger cells develop?

      Larger cells, classified 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.