The Beginning Of Life
Scientists generally agree that the first life on earth appeared sometime before 3.9 billion years ago (bya). The origins of life are known to have come after the presence of liquid water on earth. But other than that, there is no solid evidence to pin down a more precise date. However, once large pools of water had formed, it was possible for life to exist.
There is carbon isotope evidence for life in the world’s oldest known sedimentary rocks from the Isua Greenstone Belt of West Greenland estimated to be 3.85 billion years old. These carbon rich rock layers probably accumulated as plankton bacteria on the surface, died and settled to the ocean floor. These early life forms were not only alive, but capable of photosynthesis, that is inhaling carbon dioxide and exhaling oxygen.
The earliest life form was very simple. It was almost certainly cyanobacteria (mistakenly called blue-green algae at times). See the photo of a cyanobacteria bloom above. We know it existed 3.9 bya at the latest because the first life also left behind the first fossils. Assuming it took about 100 million years for life to progress to the point of photosynthesis (which is pretty sophisticated), life began at least 4.0 billion years ago.
The Latest. Distinguished UCLA professors of geochemistry, Mark Harrison and Elizabeth Bell plus some research associates, found evidence that life likely existed on earth at least 4.1 billion years ago. The researchers, led by Bell, studied more than 10,000 zircons searching for carbon, the key component for life. Zircons are heavy, durable minerals originally formed from molten rocks in Western Australia. They capture and preserve their immediate environment, which means they serve as time capsules.
The carbon contained in zircon has a characteristic signature, a specific ratio of carbon-12 to carbon-13, that indicates the presence of photo-synthetic life. The scientists identified 656 zircons containing dark specks and closely analyzed 79 of them using a technique that shows the molecular and chemical structure of ancient micro-organisms in three dimensions. One of the 79 zircons contained graphite, which is pure carbon, in two locations. The graphite is older than the zircon containing it, the researchers said. They know the zircon is 4.1 billion years old based on its ratio of uranium to lead. They don't know how much older the graphite is. This research was published in the November, 2015 issue of the Proceedings of the National Academy of Sciences. Top
Cyanobacteria And Blue-Green Algae
Cyanobacteria are a form of bacteria that obtain their energy from photosynthesis. They are frequently referred to as blue-green algae, but that is a mistake. Cyanobacteria are from the family of organisms called prokaryotes because their cells lack a nucleus surrounded by a membrane. Cyanobacteria include one cell organisms and also multiple cell species that form colonies. Colonies may form filaments, like those pictured at the left, or sheets, or hollow balls, or thick-walled micro-organisms.
Cyanobacteria are probably the most diversified group of micro-organisms on earth. They are found in a broad range of habitats from the equator to the poles. They are found in freshwater lakes, saltwater oceans, and damp soil. They are found in extreme environments such as hot springs, salt works, moistened desert rocks and in the ice-cold arctic ocean.
Cyanobacteria are known for their large, highly visible blooms that can form in both freshwater and saltwater. They have the appearance of large greenish algae blooms. These blooms are toxic, and frequently lead to the closure of recreational waters when they appear. Cyanobacteria are very important organisms for the health and growth of many plants. They are one of very few groups of organisms that can convert inert atmospheric nitrogen into an organic form, such as nitrate or ammonia. It is these fixed forms of nitrogen that plants need for their growth and must be obtained from the soil.
By producing oxygen as a by-product of photosynthesis, huge blooms of cyanobacteria over millions of years are believed to have converted the early oxygen free atmosphere into one with significant quantities of oxygen. This dramatically changed the composition of life forms on earth by stimulating biodiversity and leading to the near extinction of oxygen intolerant organisms.
Real blue-green algae are not prokaryotes, they are eukaryotes, which means they have a membrane which encloses the nucleus of their cells. Blue-green algae also have a rigid "cell wall" which makes them a plant. A eukaryote cell is shown at the left with the nucleus in pink surrounded by the membrane in yellow.
Much life on earth belongs to the eukaryote family, all the way from blue-green algae to human beings. All multicellular organisms are eukaryotes, including animals, plants and fungi. On a numerical count basis, eukaryotes represent a tiny minority of all living things. Even in the human body, there are 10 times more microbe prokaryotes than human cells.
It is believed that certain cyanobacteria evolved into blue-green algae eukaryotes about 2.5 bya (billion years ago) much, much later than when cyanobacteria first appeared. The origin of the eukaryotic cell is considered a milestone in the evolution of life since they include all complex cells and almost all multicellular organisms. It was the development of the nucleus, which allowed highly complex forms of life to eventually evolve. Top
Stromatolites - Longest Living Organisms
Stromatolites (stre' mat-o-lites') are the longest living form of life on the planet. They can be traced back at least 3.5 billion years. Current marine stromatolites are only several thousand years old and can be found in waters in Western Australia and the Bahamas. Other types of stromatolites have also been found in freshwater streams, lakes, thermal springs, and even frozen lakes. There are over 170 known types of ancient stromatolites, believed to have diversified depending on different radiation patterns and local water conditions. See the marine photo to the left from very salty Shark Bay in Australia.
Stromatolites are rock-like objects formed in shallow waters by living single celled micro-organisms, cyanobacteria, bound together in successive layers of carbonate sediment grains. (A similar building process is that of coral reefs that are alive on the edges but layers of calcium carbonate secreted by the corals on the inside.) Cyanobacteria come in population densities of over 3 billion organisms per square meter. They like very salty water and strong waves. Note the photo at the below left showing stromatolites under water, Their top third is alive while the bottom two-thirds are stone layers.
Each cyanobacteria cell secretes a sticky film of mucus that traps local sediment grains. The sediment grains are bound together by the mucus and the cyanobacteria then grows over the grains. The bacteria are mobile and they photosynthesize, so they move towards light from the sun. Because the cyanobacteria need sunlight to photosynthesize, the stromatolites are generally found in water less than six feet deep where there is considerable sunlight. Their mobility also allows them to keep up with the growing sediment layers.
The trapped sediment reacts with the calcium carbonate in the surrounding water and cements the grains together to form limestone. These limestone deposits build up very, very slowly – it can take a stromatolite 100 years to grow 2 inches. A three foot tall stromatolite might be about 1,800 years old. Without the final stage of limestone cementation, the ancient micro-organism structures would not have been preserved as fossil records.
The first records of stromatolites began about 3.5 billion years ago (bya). Their presence indicates that even at such an early age, advanced prokaryotes were present, indicating that life on earth could have began much earlier, maybe as early as 4.0 bya. Stromatolites peaked about 1.25 bya and then began to decline. Today marine stromatolites can be found only in isolated areas like Shark Bay, Australia and the Bahamas. As an example of their decline, at Lake Clifton in Western Australia, scientists are witnessing algae (eukaryotes) out competing cyanobacteria, caused by an increase in nutrient levels in the water. Top
Multicellular Organisms Appear - Red Algae
The first multicelled organisms are believed to have been red algae, which appeared sometime between 1.4 and 1.2 billion years ago. This was about two billion years after stromatolites first appeared. Thus, more than one-half the time life has been present on earth, it was occupied by only single cell organisms.
Ancient micro-fossils of red algae were preserved and have been found on Somerset Island in northern arctic Canada. These fossils are as old as 1.2 billion years. The first multicellular organisms had certain characteristics that have defined all complex life forms since. Red algae invented sex and reproduced sexually.
The male red algae releases sperm into the water which floats nearby coming into contact with the female's reproductive organ and fertilization occurs. Upon contact, the barriers dissolve inside of the female's reproductive organs. The male nucleus divides and one-half merges with the female nucleus. The female develops a large bulb which eventually buds off from the rest of the algae. This bulb is essentially a juvenile red algae which needs only time and nutrients to grow to an adult.
Sexual reproduction using egg and sperm cells is characteristic of multicelled organisms and first appeared in red algae. This development allowed much more complex life forms (including humans) to eventually evolve. So if you go back far enough, we all have red algae to thank for our existence. Top
The Cambrian Explosion
The climate at the beginning of the Cambrian Period (from 543 to 490 million years ago (mya)), was cold but as the period time passed, the climate all over the earth grew warmer. The continents were still forming and were mostly barren rocks. The land had no plant or animal life on it yet. This made the seas the preferred place for species to live. Sea levels flooded many low land masses and created shallow habitats ideal for spawning new marine life-forms.
The Cambrian Explosion lasted about 53 million years and brought about a dramatic burst of evolutionary changes in new life. Among the creatures that evolved during this period were hard-bodied clams and the ancestors of spiders and insects.
Trilobites (tri'-lo-bits), pictured at the left, were the dominant species during this period. Trilobites are extinct arthropods, animals with a hard skin shell and jointed legs. Trilobites were distant relatives of modern lobsters and horseshoe crabs. Trilobites had three (tri-lobe) segmented, rather flat, top plated bodies. They could curl up into balls for protection in seas that were increasingly filled with predators. Trilobites were the first animals to develop eyes.
Trilobites came in many varieties and sizes. They ranged from a few inches to more than 2 feet in length. Trilobites proved to be among the most successful and enduring of all prehistoric animals. More than 17,000 species are known to have existed and they survived for approximately 300 million years and then perished. A dramatic lowering of sea levels at the time probably contributed to their demise.
A dominant animal of the Cambrian Period was the giant anomalocaris, (ah-NOM'-ah-LAH'-kariss), which trapped its prey with two claw-tipped appendages lined with hooks in the front of its mouth. Anomalocaris, which means abnormal shrimp, had true compound eyes. For the time in which it lived, the anomalocaris was a gigantic creature reaching lengths of up to six feet. Anomalocaris was a free-swimming animal that undulated through the water by flexing its body like a modern dolphin. They fed on trilobites and other arthropods, worms and mollusks. Anomalocaris was the largest and most fearsome predator of the Cambrian Period.
Sponges also grew in the Cambrian seas. These animals belong to the phylum "porifera" because of all the tiny pores in their bodies. One species of sponge from this period had many branches that made it look like a tree. Another type of sponge looked like an ice cream cone without the ice cream. Many of the sponges became extinct when water temperatures dropped at the end of the Cambrian period.
The Cambrian Period ended with a mass extinction. The leading theory is that a period of continental glaciation occurred when the climate of the earth cooled at the end of the Cambrian. Scientists have suggested that the cold conditions wiped out much of the warm water organisms because they were cold intolerant. Advancing glaciers would have lowered the temperature and the levels of the shallow seas where so many marine species lived. Changes in the temperature and also the reduction of the amount of oxygen in the water would have meant the end for many species that could not readily adapt. The loss of their habitat and the increased competition among the remaining displaced species led to the demise of many of them - a truly mass extinction. Top
Plant Life On Land
About 450 million years ago (mya), soon after the Cambrian Period, plants began to make their way onto land. The first plants needed a source of water for photosynthesis, so they were found on marsh land where they could easily obtain water from the damp soil. Because they did not have any tissue that conducted water very well, they had to stay close to a supply in order to obtain the water they needed for photosynthesis.
One of the major steps in plant evolution was the widespread evolution of spores as a form of plant reproduction. Spores are unicell organisms that are mobile and can reproduce forming new plants. Because spores could migrate by wind from place to place, they allowed plants to spread across the land. Spores eventually evolved into seeds, which are the multi-cell reproduction organisms of most of our current day plants.
Another major development about 430 mya was the first appearance of vascular systems within plants. These are plant veins that circulate water, chemicals, and minerals within the plant. About 375 mya, plants that had root systems and leaves appeared for the first time. These advancements allowed plants of this era to become much larger and to function internally like plants of today. See the above artist's drawing of an early marsh environment.
As more time passed, about 300 mya, conifers appeared and thrived. Some of the trees of this family are pines, cedars, cypress, and massive redwoods. Conifers are cone bearing seed plants, mostly trees. The conifer family rapidly spread until massive conifer forests covered most of the planet. Ferns were also quite abundant as they grew well in the undergrowth of the large conifer forests. Top
Animal Life On Land - Walking Fish
For at least 1.4 billion years after the beginning of life, no animal ever treaded on land. One reason was it takes a long time for creatures to evolve from one species to another. Going from living in water to living on land was a major step and would have taken a major amount of time. Another reason might have been ultraviolet rays. For a long time, the earth did not have an ozone layer. Any creature that ventured on to land for any length of time would have been destroyed by the deadly radiation. After an oxygenated atmosphere developed, an ozone layer formed and land was safer to tread. However, the first large animals to walk the earth were probably walking fish who still lived in water. Initially there was no food on the land so there was no pressing reason for them to live there permanently.
Trace fossils are the evidence of life preserved in sediments as a result of the living activities of organisms. They include surface tracks, trails, subsurface burrows, as well as fecal material and the marks produced by dying animals. They are evidence left behind by living things, but not direct evidence of the creatures themselves. There is fossil evidence of animal tracks on land from about 530 million years ago (mya).
These tracks were probably made by tiny arthropods, animals with no spinal column (invertebrates), but have an external skeleton, a segmented body, and jointed appendages. See the photo to the left of a present day arthropod. Arthropods include flies, insects, worms, crabs, scorpions, starfish and octopus. The overwhelming majority of animal species are invertebrates. Only about 4% of all animal species have a spinal column.
It is believed that tetrapods, four limbed animals having a spinal column (vertebrates), walked on land about 400 mya according to fossil evidence. See the image to the left of an acanthostega. Tetrapods were aquatic creatures that lived in swamps and shallow ponds but ventured onto land occasionally, perhaps to mate or to hide from enemies. On land there were no enemies while in the sea there were plenty of them. Tetrapods probably also walked on the floors of their shallow swamps and ponds.
Eventually tetrapods took up permanent residence on land and survived on small insects and small plants most likely plant mats related to the green algae family. Tetrapods include amphibians, reptiles, birds, dinosaurs and mammals. The development of the vertebrate structure paved the way for more advanced animals and eventually humans. Top
How Did Life Begin?
As mentioned at the top of this page, life surely began as bacteria, most likely cyanobacteria. But how did the bacteria form and reproduce? No one really knows the answer to this question. But there are a few theories, none of which are completely compelling at this point in time.
One of the theories is that life came from elsewhere in the universe and was transported to the earth by meteors or comets. While there may be life elsewhere in the universe, not many scientists subscribe to the theory that it arrived by meteors. The heat of entry through the atmosphere is so great that it is unlikely that any form of life could survive the process. Most meteors burn up as they pass through the atmosphere and so would any form of bacteria.
However, the basic chemical ingredients for life might have arrived from outer space and then life formed here on earth. Most living organisms are made up of carbon, oxygen, hydrogen, sulfur, plus some nitrogen and phosphorous. There are a few dozen other elements in trace amounts, but as a first approximation living organisms are made of carbon, oxygen, and hydrogen.
There was a famous experiment in the early 1950s that tested the hypothesis that conditions during the primitive earth were favorable for chemical reactions to form organic compounds from inorganic ones by lightening strikes. The experiment was done by Stanley Miller and Harold Urey at the University of Chicago. Miller-Urey essentially put methane (natural gas), ammonia, hydrogen gas, and water vapor into a beaker. This was not a random mixture; at the time they did the experiment, this mixture made up what was thought to be the early atmosphere.
They put an electric charge through the mixture to simulate lightning striking through the early atmosphere. After the experiment ran for a few days, all of a sudden there was brown goo all over the reaction vessel. When they analyzed what was in the vessel, they found amino acids which are the building blocks of proteins. In fact, they occurred in just about the same proportions one would find if you looked at organic matter in a meteorite.
So the chemistry that Miller-Urey used in this experiment was not some improbable chemistry, but a chemistry that is widely distributed throughout the solar system. This famous experiment lent support to the theory that the composition of the early earth and its atmosphere could have been the result of organic molecules being formed by nature itself. Subsequent experiments with different starting chemicals have yielded different amino acids and other compounds, but none have shown any form of life.
So what can be tentatively concluded is that "life is a form of chemistry", a particular form in which chemicals can evolve to perform their own reproduction. When we think about the origin of life this way, it isn't that life is somehow different from the rest of the planet. Life is something that emerges on a developing planet's surface as part and parcel of the normal chemistry of that surface. All life that we know is fundamentally very similar. If you look at the cell of a bacterium, it has about the same proportions of carbon, oxygen and hydrogen as a human body does. The basic biochemical machinery of a bacterium is in a way similar to the chemistry of our human cells. While we do not know the precise mechanisms of how life began, we now know it was not something highly unusual, but was part of the normal development of a planet under favorable conditions.