The First Life on Earth
Earth was able to support life only after the planet had cooled enough for a rocky crust to solidify. Once that happened, water vapor from volcanoes condensed in the atmosphere, fell as rain, and collected on the Earth’s surface. Besides water vapor, volcanoes also produced gases rich in the basic ingredients of life: carbon, hydrogen, oxygen, and nitrogen. Toxic gases such as ammonia and methane were common. At this point, Earth's early atmosphere consisted entirely of these volcanic gases, and there was no free oxygen. In the primordial “soup” of the early seas, organic molecules concentrated, formed more complex molecules, and became simple cells.
The transition from complex organic molecules to living cells could have occurred in several environments. Small, warm ponds are one possibility, but recent work has suggested that deep-sea hydrothermal vents, such as those found along mid-ocean spreading centers today, may have been the cradle of Earth's life. These environments contain the chemicals and the source of energy needed to synthesize more complex organic structures. Although scientists have not succeeded in creating life from organic molecules in the laboratory, they have reproduced many of the intermediate steps.
So what were the first living things and when did they appear? Studies of genetic material indicate that a living group of single-celled organisms called Archaea may share many features with early life on Earth. Many Archaea now live in hot springs, deep-sea vents, saline water, and other harsh environments. If the first organisms resembled modern Archaea, they also may have lived in such places, but direct evidence for early life is controversial because it is difficult to distinguish between complex inorganic structures and simple biological ones in the geologic record. The oldest evidence for life may be 3.5-billion-year-old sedimentary structures from Australia that resemble stromatolites.Stromatolites are created today by living mats of microorganisms (mostly cyanobacteria, or blue-green algae). These primitive organisms trap thin layers of sediment with their sticky filaments and grow upward to get light for photosynthesis. Modern-day examples of stromatolites can be found in waters off Australia, the Bahamas, and Belize.
In the Archean structures, layers similar to those seen in living stromatolites are evident, and secondary structures interpreted as simple filamentous microfossils have been recovered from the layers. The biotic origin of the structures has, however, been questioned. Both the supposed Archean stromatolites and the microfossils may have been produced by inorganic processes. Regardless, uncontested microfossils and chemical traces of life were present at least by 2.7 billion years ago. Stromatolites that were produced by microorganisms are abundant later in the Archean and throughout the Proterozoic. These sedimentary structures, formed by organic processes, provide important evidence of early life. At present, we can say with certainty that life had evolved by 2.7 billion years ago, and possibly as early as 3.5 billion years ago.
Life on Earth has been changing at various rates since our common ancestor first appeared more than 3.5 billion years ago. To better understand the changes that have taken place, it helps to look for milestones in the history of life on Earth. By grasping how organisms, past and present, have evolved and diversified throughout the history of our planet, we can better appreciate the animals and wildlife that surround us today.
The first life evolved more than 3.5 billion years ago. Scientists estimate that the Earth is some 4.5 billion years old. For nearly the first billion years after the Earth formed, the planet was inhospitable to life. But by about 3.8 billion years ago, the Earth's crust had cooled and the oceans had formed and conditions were more suitable for the formation of life. The first living organism formed from simple molecules present in the Earth's vast oceans between 3.8 and 3.5 billion years ago. This primitive lifeform is know as the common ancestor. The common ancestor is the organism from which all life on Earth, living and extinct, descended.
Photosynthesis arose and oxygen began accumulating in the atmosphere about 3 billion years ago. A type of organism known as cyanobacteria evolved some 3 billion years ago. Cyanobacteria are capable of photosynthesis, a process by which energy from the sun is used to convert carbon dioxide into organic compounds—they could make their own food. A byproduct of photosynthesis is oxygen and as cyanobacteria persisted, oxygen accumulated in the atmosphere.
Sexual reproduction evolved about 1.2 billion years ago, initiating a rapid increase in the pace of evolution. Sexual reproduction, or sex, is a method of reproduction that combines and mixes traits from two parent organisms in order to give rise to an offspring organism. Offspring inherit traits from both parents. This means that sex results in the creation of genetic variation and thus offers living things a way to change over time—it provides a means of biological evolution.
The Cambrian Explosion is the term given to the time period between 570 and 530 million years ago when most modern groups of animals evolved. The Cambrian Explosion refers to an unprecedented and unsurpassed period of evolutionary innovation in the history of our planet. During the Cambrian Explosion, early organisms evolved into many different, more complex forms. During this time period, nearly all of the basic animal body plans that persist today came into being.
The first back-boned animals, also known as vertebrates, evolved about 525 million years ago during the Cambrian Period. The earliest known vertebrate is thought to be Myllokunmingia, an animal that is thought to have had a skull and a skeleton made of cartilage. Today there are about 57,000 species of vertebrates that account for about 3% of all known species on our planet. The other 97% of species alive today are invertebrates and belong to animal groups such as sponges, cnidarians, flatworms, molluscs, arthropods, insects, segmented worms, and echinoderms as well as many other lesser-known groups of animals.
The first land vertebrates evolved about 360 million years ago. Prior to about 360 million years ago, the only living things to inhabit terrestrial habitats were plants and invertebrates. Then, a group of fishes know as the lobe-finned fishes evolved the necessary adaptations tomake the transition from water to land.
Between 300 and 150 million years ago, the first land vertebrates gave rise to reptiles which in turn gave rise to birds and mammals. The first land vertebrates were amphibioustetrapods that for some time retained close ties with the aquatic habitats they had emerged from. Over the course of their evolution, early land vertebrates evolved adaptations that enabled them to live on land more freely. One such adaptation was the amniotic egg. Today, animal groups including reptiles, birds and mammals represent the descendents of those early amniotes.
The genus Homo first appeared about 2.5 million years ago. Humans are relative newcomers to the evolutionary stage. Humans diverged from chimpanzees about 7 million years ago. About 2.5 million years ago, the first member of the genus Homo evolved, Homo habilis. Our species, Homo sapiens evolved about 500,000 years ago.
How Did Life Start On Earth?
Scientists do not know how life began on Earth. They do know that the early Earth’s atmosphere was very different from the atmosphere now.
In 1952, Stanley Miller was working with Harold C. Urey designed an experiment to see how complex organic molecules might have formed under the conditions of early Earth. They believed the early Earth atmosphere would have been composed of methane, ammonia, hydrogen and water vapor. They sealed these gases in an airtight container, and then exposed the gases to sparks of electricity to simulate lightning. They continued the lightning for a week, and by the end, a reddish-brown substance had coated the walls of the container. This substance contained 11 of the 20 amino acids used by life on earth. Since Miller and Urey performed this experiment, its results have been confirmed many times by other scientists. Many scientists now believe that the early Earth’s atmosphere was composed of carbon dioxide, nitrogen and water vapor.
Modern experiments with this mixture of gases produce similar results suggesting that early conditions on Earth produced complex organic molecules that probably became the basis for the development of more complex organisms. However, scientists have not been able to replicate the formation of even simple organisms, or anything that can really replicate itself. There are several theories as to how the amino acids might have made the leap into the complex, self-replicating life we see today.
Metabolism - First
Some scientists believe that metabolism, in other words - the ability to break down carbon dioxide in the presence of a catalyst into small organic molecules - was how the first life developed. These reactions might have evolved to become more complex, and then genetic molecules somehow formed and joined in later. There are many different theories as to exactly what types of molecules and catalysts would have been involved.
Genes - First
Other scientists believe that the first living organisms were genes. These genes were single molecules that had developed in such a way as to be able to catalyze their own replication. This theory seems more likely, since even simple systems such as crystals, have been demonstrated to evolve with modifications that breed true. Some scientists have suggested that certain compositions of clay create the right environment for these reactions to propagate.
RNA
RNA is a complex molecule found in all living things that seems to be able to catalyze its own reproduction. Many scientists believe that simple RNA molecules developed and eventually became more complex and developed into the organisms we see today.
LUCA
Astrobiologists and biochemists want to understand something they call LUCA (the Last Universal Common Ancestor). The idea is that all life on Earth has a common ancestor, kind of like a great-great-great-....-great grandmother. They search for traits that are common across all life forms and assume that any traits that are common to all life forms today must have been inherited from LUCA, who had them all as well.
Biochemists know quite a bit about LUCA and her biochemistry. She stored her genetic information in DNA, she had several hundred proteins performing a variety of functions, and she used the same 20 amino acids we use in our proteins. She used RNA and had some kind of double-layer lipid membrane. She was probably the ancestor of the three kingdoms of life: Archaea, Eukaryotes and Bacteria.
LUCA lived at least 2 billion years ago, before there was much oxygen in the atmosphere. She used enzymes containing iron in her metabolic pathways the way much life on early Earth did. Studying how life arose on Earth is useful to astrobiologists, but they keep in mind that the way life formed on Earth is not the only way life could have formed. It is simply one way that it did.
Earth was able to support life only after the planet had cooled enough for a rocky crust to solidify. Once that happened, water vapor from volcanoes condensed in the atmosphere, fell as rain, and collected on the Earth’s surface. Besides water vapor, volcanoes also produced gases rich in the basic ingredients of life: carbon, hydrogen, oxygen, and nitrogen. Toxic gases such as ammonia and methane were common. At this point, Earth's early atmosphere consisted entirely of these volcanic gases, and there was no free oxygen. In the primordial “soup” of the early seas, organic molecules concentrated, formed more complex molecules, and became simple cells.
The transition from complex organic molecules to living cells could have occurred in several environments. Small, warm ponds are one possibility, but recent work has suggested that deep-sea hydrothermal vents, such as those found along mid-ocean spreading centers today, may have been the cradle of Earth's life. These environments contain the chemicals and the source of energy needed to synthesize more complex organic structures. Although scientists have not succeeded in creating life from organic molecules in the laboratory, they have reproduced many of the intermediate steps.
So what were the first living things and when did they appear? Studies of genetic material indicate that a living group of single-celled organisms called Archaea may share many features with early life on Earth. Many Archaea now live in hot springs, deep-sea vents, saline water, and other harsh environments. If the first organisms resembled modern Archaea, they also may have lived in such places, but direct evidence for early life is controversial because it is difficult to distinguish between complex inorganic structures and simple biological ones in the geologic record. The oldest evidence for life may be 3.5-billion-year-old sedimentary structures from Australia that resemble stromatolites.Stromatolites are created today by living mats of microorganisms (mostly cyanobacteria, or blue-green algae). These primitive organisms trap thin layers of sediment with their sticky filaments and grow upward to get light for photosynthesis. Modern-day examples of stromatolites can be found in waters off Australia, the Bahamas, and Belize.
In the Archean structures, layers similar to those seen in living stromatolites are evident, and secondary structures interpreted as simple filamentous microfossils have been recovered from the layers. The biotic origin of the structures has, however, been questioned. Both the supposed Archean stromatolites and the microfossils may have been produced by inorganic processes. Regardless, uncontested microfossils and chemical traces of life were present at least by 2.7 billion years ago. Stromatolites that were produced by microorganisms are abundant later in the Archean and throughout the Proterozoic. These sedimentary structures, formed by organic processes, provide important evidence of early life. At present, we can say with certainty that life had evolved by 2.7 billion years ago, and possibly as early as 3.5 billion years ago.
Life on Earth has been changing at various rates since our common ancestor first appeared more than 3.5 billion years ago. To better understand the changes that have taken place, it helps to look for milestones in the history of life on Earth. By grasping how organisms, past and present, have evolved and diversified throughout the history of our planet, we can better appreciate the animals and wildlife that surround us today.
The first life evolved more than 3.5 billion years ago. Scientists estimate that the Earth is some 4.5 billion years old. For nearly the first billion years after the Earth formed, the planet was inhospitable to life. But by about 3.8 billion years ago, the Earth's crust had cooled and the oceans had formed and conditions were more suitable for the formation of life. The first living organism formed from simple molecules present in the Earth's vast oceans between 3.8 and 3.5 billion years ago. This primitive lifeform is know as the common ancestor. The common ancestor is the organism from which all life on Earth, living and extinct, descended.
Photosynthesis arose and oxygen began accumulating in the atmosphere about 3 billion years ago. A type of organism known as cyanobacteria evolved some 3 billion years ago. Cyanobacteria are capable of photosynthesis, a process by which energy from the sun is used to convert carbon dioxide into organic compounds—they could make their own food. A byproduct of photosynthesis is oxygen and as cyanobacteria persisted, oxygen accumulated in the atmosphere.
Sexual reproduction evolved about 1.2 billion years ago, initiating a rapid increase in the pace of evolution. Sexual reproduction, or sex, is a method of reproduction that combines and mixes traits from two parent organisms in order to give rise to an offspring organism. Offspring inherit traits from both parents. This means that sex results in the creation of genetic variation and thus offers living things a way to change over time—it provides a means of biological evolution.
The Cambrian Explosion is the term given to the time period between 570 and 530 million years ago when most modern groups of animals evolved. The Cambrian Explosion refers to an unprecedented and unsurpassed period of evolutionary innovation in the history of our planet. During the Cambrian Explosion, early organisms evolved into many different, more complex forms. During this time period, nearly all of the basic animal body plans that persist today came into being.
The first back-boned animals, also known as vertebrates, evolved about 525 million years ago during the Cambrian Period. The earliest known vertebrate is thought to be Myllokunmingia, an animal that is thought to have had a skull and a skeleton made of cartilage. Today there are about 57,000 species of vertebrates that account for about 3% of all known species on our planet. The other 97% of species alive today are invertebrates and belong to animal groups such as sponges, cnidarians, flatworms, molluscs, arthropods, insects, segmented worms, and echinoderms as well as many other lesser-known groups of animals.
The first land vertebrates evolved about 360 million years ago. Prior to about 360 million years ago, the only living things to inhabit terrestrial habitats were plants and invertebrates. Then, a group of fishes know as the lobe-finned fishes evolved the necessary adaptations tomake the transition from water to land.
Between 300 and 150 million years ago, the first land vertebrates gave rise to reptiles which in turn gave rise to birds and mammals. The first land vertebrates were amphibioustetrapods that for some time retained close ties with the aquatic habitats they had emerged from. Over the course of their evolution, early land vertebrates evolved adaptations that enabled them to live on land more freely. One such adaptation was the amniotic egg. Today, animal groups including reptiles, birds and mammals represent the descendents of those early amniotes.
The genus Homo first appeared about 2.5 million years ago. Humans are relative newcomers to the evolutionary stage. Humans diverged from chimpanzees about 7 million years ago. About 2.5 million years ago, the first member of the genus Homo evolved, Homo habilis. Our species, Homo sapiens evolved about 500,000 years ago.
How Did Life Start On Earth?
Scientists do not know how life began on Earth. They do know that the early Earth’s atmosphere was very different from the atmosphere now.
In 1952, Stanley Miller was working with Harold C. Urey designed an experiment to see how complex organic molecules might have formed under the conditions of early Earth. They believed the early Earth atmosphere would have been composed of methane, ammonia, hydrogen and water vapor. They sealed these gases in an airtight container, and then exposed the gases to sparks of electricity to simulate lightning. They continued the lightning for a week, and by the end, a reddish-brown substance had coated the walls of the container. This substance contained 11 of the 20 amino acids used by life on earth. Since Miller and Urey performed this experiment, its results have been confirmed many times by other scientists. Many scientists now believe that the early Earth’s atmosphere was composed of carbon dioxide, nitrogen and water vapor.
Modern experiments with this mixture of gases produce similar results suggesting that early conditions on Earth produced complex organic molecules that probably became the basis for the development of more complex organisms. However, scientists have not been able to replicate the formation of even simple organisms, or anything that can really replicate itself. There are several theories as to how the amino acids might have made the leap into the complex, self-replicating life we see today.
Metabolism - First
Some scientists believe that metabolism, in other words - the ability to break down carbon dioxide in the presence of a catalyst into small organic molecules - was how the first life developed. These reactions might have evolved to become more complex, and then genetic molecules somehow formed and joined in later. There are many different theories as to exactly what types of molecules and catalysts would have been involved.
Genes - First
Other scientists believe that the first living organisms were genes. These genes were single molecules that had developed in such a way as to be able to catalyze their own replication. This theory seems more likely, since even simple systems such as crystals, have been demonstrated to evolve with modifications that breed true. Some scientists have suggested that certain compositions of clay create the right environment for these reactions to propagate.
RNA
RNA is a complex molecule found in all living things that seems to be able to catalyze its own reproduction. Many scientists believe that simple RNA molecules developed and eventually became more complex and developed into the organisms we see today.
LUCA
Astrobiologists and biochemists want to understand something they call LUCA (the Last Universal Common Ancestor). The idea is that all life on Earth has a common ancestor, kind of like a great-great-great-....-great grandmother. They search for traits that are common across all life forms and assume that any traits that are common to all life forms today must have been inherited from LUCA, who had them all as well.
Biochemists know quite a bit about LUCA and her biochemistry. She stored her genetic information in DNA, she had several hundred proteins performing a variety of functions, and she used the same 20 amino acids we use in our proteins. She used RNA and had some kind of double-layer lipid membrane. She was probably the ancestor of the three kingdoms of life: Archaea, Eukaryotes and Bacteria.
LUCA lived at least 2 billion years ago, before there was much oxygen in the atmosphere. She used enzymes containing iron in her metabolic pathways the way much life on early Earth did. Studying how life arose on Earth is useful to astrobiologists, but they keep in mind that the way life formed on Earth is not the only way life could have formed. It is simply one way that it did.
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