Chapter 17. The planet Earth

17.1 Characteristics of Earth

Earth is the third closest planet to the Sun, and completes one orbit every 365.25 days. Earth is larger, denser, and more massive than Mercury, Venus, and Mars, and has one natural satellite, the Moon.[1]

Earth is the only place in the universe where life is known to exist, and almost all life on Earth is fuelled by energy from the Sun. It takes light just over 8 minutes to travel to Earth, across a distance of about 150 million km. This distance is defined as one Astronomical Unit (AU).

Over 70% of the surface of the Earth is covered with salt-water oceans. The rest is composed of continents and islands. The interior of the Earth contains a thick layer of solid mantle, a liquid outer core that generates a magnetic field, and a solid inner core of iron. The Earth’s axis is tilted by about 23.4°, and this is what causes us to experience seasons (discussed in Chapter 2).

Shortly after its formation, about 4.54 billion years ago, the Earth’s atmosphere was primarily composed of hydrogen and helium. Hydrogen and helium are the least massive atoms (discussed in Book II), and so were able to escape the gravitational field of the Earth. These were soon replaced by heavier molecules like water, carbon dioxide, and ammonia, which were released from the Earth by volcanoes.[2] During the Earth’s first 700 million years, the surface was bombarded with comets, which brought water and other elements and molecules, including amino acids, the ‘building blocks of life’.[3]

17.2 Life on Earth

17.2.1 The first life

We still don’t know how life came into existence on Earth, although all life forms are built from amino acids, which can arise naturally.[4,5] The first life consisted of prokaryotes, simple cells that may have evolved about 3.8 billion years ago, within a billion years of the formation of the Earth.[3] Around this time, a common ancestor gave rise to two groups: bacteria and archaea.[6] There’s evidence that viruses have existed for at least 3 billion years,[7] although viruses are not generally considered to be alive.[8]

Photograph of astronaut Mark C. Lee above the Earth.

Figure 17.1
Image credit

Astronaut Mark C. Lee above the Earth.

During this time, the atmosphere of the Earth was mostly composed of carbon dioxide, water, and molecular nitrogen. This is thought to have rapidly changed about 2.5 billion years ago, when photosynthesising bacteria began to excrete oxygen for the first time, poisoning almost all life that had evolved before. This is known as ‘the great oxidation event’.[9]

Over 1.5 billion years ago, one bacterial cell engulfed another, and eukaryotic cells were formed.[10] Eukaryotic cells contain nuclei and store genes in the form of DNA. The engulfed bacteria eventually became mitochondria, which provide the cell with energy. Eukaryotic cells later engulfed photosynthetic bacteria and evolved into chloroplasts, which are found in green algae and green plants.[11] Within a few hundred million years, eukaryotes divided into three groups - the ancestors of plants, fungi, and animals.[12]

Multicellular life may have developed up to 2 billion years ago.[13] Sponges evolved about 800 million years ago,[14] and the Cambrian explosion began about 530 million years ago.[15] Many new species evolved during this time, including the first animal with a backbone and the first trilobites. Trilobites had primitive eyes, gills, limbs, and a simple brain. Animals moved from the sea to the land about 500 million years ago,[16] and plants followed within about 25 million years.[17]

Photograph of a lion.

Figure 17.2
Image credit

Life on Earth: a lion.

Photograph of a springbok.

Figure 17.3
Image credit

Life on Earth: a springbok.

The genetic code

If you look at any of the different parts of an animal through a microscope, their bones, or organs, or muscles, for example, then you will find that they are all composed of tissue, which is created from cells. These cells can have different functions, but they all have a nucleus that contains the DNA of the animal they belong to.[18] Single strands of DNA can wrap into a structure known as a chromosome, which we can see with electron microscopes. If you zoom in on a chromosome, you will see that a strand of DNA looks like it’s actually made of two strands, held together by bars. These bars are composed of pairs of chemicals known as nucleobases.

There are four types of nucleobases - adenine (A), thymine (T), cytosine (C), and guanine (G). A can only pair with T, and G can only pair with C. This means that each half of a strand of DNA contains enough information to replicate itself. If one half has an A nucleobase, for example, then the other half must be T. DNA replicates within the nucleus of cells by splitting into two strands. These then make up the two halves of two new strands, and the rest of the structure is built around them.

Almost all of an animal’s physical characteristics are determined by proteins, which are made from chains of amino acids, and DNA contains information on which proteins to make. This information is encoded in the order of nucleobases, which are read in groups of three, along one-half of the DNA strand.[18] The order G, C, A, for example, is code for the amino acid alanine, and A, G, A is code for the amino acid arginine, and so the code G, C, A, A, G, A means make a protein chain starting with the amino acid alanine followed by arginine. Each set of three is known as a codon, and there are 64 possible codons, including codons stating where a chain begins and ends. The whole chain of codons, corresponding to a whole chain of amino acids needed to make a protein, is known as a gene.

A single strand of DNA forms a chromosome, and different species have a different number of chromosomes. Humans have 23 different types of chromosomes contained in the nucleus of their cells. Humans are the same because our DNA is composed of genes that are arranged in the same order - in order of the tasks they perform - along the same number of chromosomes. We are different because we can have different genes perform the same tasks.

The gene HERC2 on the 15th human chromosome, for example, designates eye colour in humans, but humans have different eye colours, and so the code for this gene can differ from individual to individual. The difference in just one nucleobase, for example, results in the difference between a gene for blue eyes and a gene for brown eyes.

After each strand of DNA has been replicated, the cell can then divide, in a process known as mitosis.[18] The production of egg and sperm cells are slightly different, and are formed in a process known as meiosis. Here, after the strands of DNA have replicated, sections of DNA on each chromosome, and its partner, are swapped. This process mixes traits inherited from the chromosomes provided by both of a person’s biological parents. The cells then divide again, with chromosome pairs separating, so that each resulting egg or sperm cell only contains a single set of 23 chromosomes. The mixing of chromosomes means that the egg and sperm cells in each person are all different. When an egg is fertilised, it inherits one set of chromosomes from each biological parent, resulting in a somatic cell with 46 chromosomes.[18] The resulting child will then have genes taken from all four of its biological grandparents. Identical twins are created if the fertilised egg then splits, if one half splits again, then identical triplets will be produced, and so on. The Human genome,[19] the Neanderthal genome,[20] and the genome of many other animals[21] were recently mapped.

17.2.2 Mass extinction events

There have been at least five mass extinction events on Earth, starting with the Ordovician event, which occurred about 440 million years ago and may have been due to an ice age. About 86% of species were wiped out.[22] Tetrapods, the first four legged animals, evolved within 40 million years of the Ordovician event. These are the ancestors of all amphibians, reptiles, birds, and mammals.[23]

The second mass extinction, the Devonian event, occurred about 360 million years ago, and about 75% of species were wiped out.[22] The third, and most devastating mass extinction, the Permian mass extinction, occurred about 250 million years ago. About 96% of species were wiped out.[22] It’s not clear what caused these events, but they may be linked to comet or asteroid impacts.

The first dinosaurs evolved about 225 million years ago, and they became dominant after a fourth mass extinction, the Triassic-Jurassic mass extinction. This occurred about 200 million years ago, and may have been due to volcanic activity. About 80% of species were wiped out.[22]

Mammals and marsupials diverged about 150 million years ago.[24] Plants began to flower about 130 million years ago,[25] and grass evolved about 70 million years ago.[26]

The fifth mass extinction event, the Cretaceous event, occurred about 65 million years ago, and wiped out 76% of species, including many species of dinosaur.[22] This was most probably due to the Chicxulub object, which may have been an asteroid or a comet.[27]

The Chicxulub object was about 10 km wide and struck the Yucatan Peninsula in Mexico about 65 million years ago. The resulting explosion is thought to have released the same amount of energy as 100,000 billion tonnes of TNT, making it 2 million times more powerful than the most powerful hydrogen bomb ever detonated.[28]

The Chicxulub object is thought to have landed on a bed of sulfur, 10 metres below the surface of the ocean. The atmosphere contained more oxygen than it does today, and so the sky was more combustible. It set alight, mixing with the sulfur to create a rain of sulfuric acid.

Light from the Sun was blocked out, and so there was very little photosynthesis on Earth for several months. This meant that most of the creatures that survived were those in food chains dependent on dead plant material - detritus.[29] The remaining dinosaurs evolved into modern birds, and mammals began to prosper.

Painting showing a large spherical rock entering the Earth’s atmosphere.

Figure 17.4
Image credit

Artist’s impression of an impact event on Earth.

Photograph of plant-life by the sea in South Africa.

Figure 17.5
Image credit

Plant-life on Earth.

Photograph of plant-life in South Africa.

Figure 17.6
Image credit

Plant-life on Earth.

Impact and extinction cycles

Over 180 impact craters have been identified on Earth, and most of these were discovered in the first half of the 20th century.[30] Impacts have been associated with mass extinction events since the 1980s,[31] with strong evidence coming from the Chicxulub Crater, which was linked to the extinction of most of the dinosaurs about 65 million years ago.[32]

Mass extinction events were soon shown to be periodic,[33] and this led astronomers to search for periodicities in impact events.[34] Both were shown to have periods of about 26 million years.[35-37]

The latest impact event occurred about 12 million years ago, and so if this cycle is correct, there should be another impact event, followed by another mass extinction, in about 14 million years.

It’s still not known why impact events appear to be cyclical. The most likely suggestions are that they are caused by a massive undetected object in the Solar System, or by the Sun’s movement through the Galaxy.

Planet X

In the 1980s, it was suggested that impact events occur in cycles because of an undetected object in the Solar System that periodically passes close to the Oort Cloud (discussed in Chapter 26), resulting in comet showers throughout the Solar system.[38] This object was first thought to be another star, referred to as ‘Nemesis’ or the ‘Death Star’.

This has since been disproved, however it’s still possible that there’s a planet-sized object in the Oort Cloud or Kuiper Belt that periodically causes impact events. This object is referred to as Planet X. Astronomer Mike Brown and planetary scientist Konstantin Batygin have recently shown that there might be a planet at the edge of the Kuiper Belt that is around 10 times the mass of the Earth,[39] and astronomers are now looking for further evidence of this.

The Sun’s orbit through the Galaxy

Other early suggestions involved the Sun’s path through the Galaxy. It was first suggested that the Oort Cloud was affected as the Sun moves through the gravitational centre of the Galactic disc (discussed in Chapter 10). One problem with this idea, however, is that scientists think we last moved though the centre of the Galactic disc about 1 million years ago,[40] yet the last impact event was about 11 million years ago.

A dark matter disc

Physicists Lisa Randall and Matthew Reece showed that the Oort Cloud may be affected in a similar way if it passes through a thin disc of dark matter.[41] If this happens, then it may also capture some of the dark matter, which could form a ball at the centre of the Earth’s core, causing the core to increase in temperature.[42]

This may lead to an increase in volcanic activity and earthquakes, and could even cause the Earth’s magnetic poles to reverse, so that the North Magnetic Pole becomes the South Magnetic Pole, and vice versa.

There’s currently no strong evidence for a dark matter disc around the Milky Way, however Randall and Reece’s predictions can be tested by the ESA’s Gaia satellite, which is currently mapping the gravitational field of the Galaxy. If a dark matter disc is detected, then this would mean that the geological and biological evolution that has taken place on Earth is directly linked to the distribution of matter in the Galaxy.

17.2.3 Hominids

Over 99% of our evolutionary history follows the same line as chimpanzees, with our ancestors diverging about 6 million years ago.[43] About 4 million years ago, a new species of hominid, or great ape, Australopithecus, evolved in the tropical forests of Africa.[44] Australopithecus were one of the first hominids to walk upright on two legs,[45] and possibly the first to create tools out of stone.[46] They became the dominant species of hominid until Homo habilis evolved about 2.4 million years ago.[47]

The skulls of Homo habilis revel that they had larger brains than the Australopithecus, and it’s thought that they were better equipped to learn by imitation.[48] Within a million years, Australopithecus had vanished. Almost 1.7 million years ago, another species, Homo erectus, evolved, possibly directly from habilis.[49] Their lives overlapped until just under 1.4 million years ago, when habilis died out.

Homo erectus explored the Earth, and their fossilised bones have been found in Africa, Europe, Indonesia, and China.[50] They were also thought to hunt, to use fire, to make complex tools, and to build campsites.[51] They may have looked after each other when they were weak or frail,[52] and may have begun to develop simple language skills.[53] It’s possible that language developed about 300,000 years ago. Homo erectus died out about 150,000 years ago.

17.2.4 Homo sapiens

The void left by Homo erectus was filled by two new types of hominid, Homo neanderthalensis, Neanderthals, and Homo sapiens, human beings. Neanderthals evolved about 400,000 years ago[54] and Homo sapiens evolved about 200,000 years ago.[55] Humans and Neanderthals were able to reproduce, and began interbreeding about 100,000 years ago.[56]

Neanderthals proved to be tougher than humans in some respects, living in colder areas, occupied by cave lions, cave bears, woolly rhinos, and woolly mammoths, while most humans were still living in a tropical climate.[57,58] The brains of Neanderthals were just as large as the brains of humans. They made tools, clothes, and jewellery, and there is evidence that they placed flowers in the graves of their dead,[59] but they couldn’t master the same long-range weapons that the humans had.[60] Neanderthals died out about 30,000 years ago.

The first dogs may have been domesticated about 35,000 years ago.[61] By this time, humans had begun creating images, and by about 15,000 BCE, humans had created paintings, drawings, engravings, sculptures, and ceramics.[62]

Humans began farming in about 10,000 BCE, and towns and then cities began to develop as people formed larger and larger groups. The city of Jericho, in the West Bank, formed in about 9,000 BCE, and cows were domesticated in Africa and the Middle East in about 8,000 BCE, around the time most woolly mammoths became extinct. Within 500 years, the city of Çatalhöyük formed in Turkey, and wheat was cultivated in the Middle East.[63]

Sheep were domesticated in the Middle East in about 7,000 BCE, and rice was cultivated in China. Chickens were domesticated in about 6,000 BCE in Southeast Asia.[63] Finally, people began to develop a written language in about 3000 BCE.[64]

Although humans appear to dominate the Earth, we’re still living in an age of bacteria. There are thought to be more bacteria, by weight, than all other life forms combined,[65] and humans contain at least as many bacterial cells as human cells.[66]

Humans have had a significant effect on the environment since the industrial revolution, which began in the late 1700s. This is because the industrial revolution led to an increase in amount of coal that was burnt, and this produces carbon dioxide. Carbon dioxide is too heavy to escape into space and so builds up, forming a layer in the atmosphere that traps the light of the Sun.[67] This is causing the average temperature of the Earth to rise.[68]

As the temperature of the Earth increases, glaciers will melt, there will be more extreme weather events such as droughts and heavy rainfall,[69] and many species will be wiped out.[70] It’s hoped that human intervention can prevent this from happening.[71]

NASA observes the Earth using the Earth Observing System. This is composed of a series of satellites that orbit, and monitor the Earth. The first of these satellites was launched in 1997, and there are currently about 22 satellites in operation.[72]

Depiction of satellites currently working as part of the Earth Observing System.

Figure 17.7
Image credit

Satellites in the Earth Observing System.

Earth Fact Sheet[1]

Designation = Terrestrial (rocky) planet
Mass = 5.97×1024 kg
Radius = 6371 km
Density = 5514 kg/m3
Length of Day = 24 hours
Length of year = 365.25 Earth-days
Distance from the Sun = 1.5×108 km (1 AU)
Orbital Velocity = 29.8 km/s
Orbital Eccentricity = 0.017
Obliquity (tilt) = 23.4°
Mean Temperature = 15 °C
Moons = 1 (the Moon)
Ring System = None

17.3 Missions to space

Life forms - consisting of fruit flies, rye, and cotton seeds - were first launched into space by the US Air Force in 1947.[73] This was just two years after World War II, and less than 50 years since the Wright brothers built the first aeroplane. The fruit flies were recovered alive, and a year later the US Air Force launched the first mammal into space, an anaesthetised rhesus macaque monkey called Albert.[74]

Albert died of suffocation before he reached the 100 km mark that’s internationally accepted as the beginning of space. Albert II became the first monkey to pass this barrier in 1949. Unlike Albert I, he survived the flight but died on impact. Six Alberts were launched into space between 1948 and 1951, and all of them died as a result.[74]

In 1952, Patricia and Mike became the first monkeys to survive a flight to space, although they only travelled 26 km above the Earth. They went on to live at the National Zoological Park in Washington, DC. Between 1958, the year that NASA was formed, and 1961, the year that the first human travelled to space, the US launched at least 18 mice and five more monkeys. All of the mice and at least one of the monkeys died as a result.[75]

While the US Air Force experimented with monkeys, the USSR Academy of Sciences preferred to use dogs. This was because they thought they coped better during long periods of confinement and inactivity.[76]

Between 1951 and 1957, the Soviet Union launched at least nine dogs into space, five of which died.[75] All of the dogs were stray female mongrels. Females were chosen because they don’t need extra room to cock their leg when urinating. As part of their training, the dogs were put in centrifuges, which simulated the high g-force of take-off, and were confined to small boxes for up to 20 days at a time.[77]

In 1957, Sputnik 1 became the first artificial satellite to orbit the Earth. The second, Sputnik 2, followed within a month, carrying Laika, the first dog - and the first animal - to be put into orbit. Laika was destined to die in space, as no one knew how to bring her home alive. The Soviet Union announced, at the time, that Laika had died painlessly after spending 7 days in space. The truth - that she had died on the first day when her capsule overheated - did not come out until 2002.[78]

Oleg Gazenko, the Soviet scientist who selected and trained Laika expressed his regret. In 1998, he stated:

“The more time passes, the more I’m sorry about it. We did not learn enough from the mission to justify the death of a dog”.[78]

At least 16 more dogs were launched between 1958 and 1966, and fatality rates dramatically decreased, with only two dying.[74] The first animals were successfully returned from orbit in 1960, these included the dogs Belka and Strelka, a rabbit, 40 mice, two rats, and numerous fruit flies.[75]

All of these sacrifices taught us that mammals could survive the effects of space travel. In 1961, NASA successfully launched our closest living relative, the chimpanzee, into space, and three months later, both countries felt confident enough to try launching a person into space. Russian cosmonaut Yuri Gagarin became the first person to travel to space on 12th April 1961, when he spent 108 minutes orbiting the Earth in the Vostok 1 spacecraft, before safely landing back on Earth. Less than a month later, American astronaut Alan Shepard became the second person to travel to space.[74]

Over 500 people have visited space since Gagarin’s pioneering mission, twelve of which walked on the Moon[79] (discussed in Chapter 18). Although it’s more cost effective to explore space using robotic probes, we will need to learn how humans can live in such environments if we ever want to colonise the Solar System.

After the race to the Moon had been won, both countries turned their attention to building space stations, where they could conduct long-term biological experiments on all forms of life. The Soviet Union utilised the Salyut program, which consisted of nine single-module stations launched between 1971 and 1982. The first American space station, known as Skylab, orbited from 1973 to 1979. These were followed by the Soviet Union’s Mir, which remained in orbit from 1986 to 2001, and the International Space Station (ISS), which launched in 1998, and should remain in orbit until at least 2024.

Animals were better cared for after NASA appointed their first Chief Veterinary Officer, Joe Bielitzki, in the 1990s. Bielitzki established a code of ethical guidelines, which states that all of NASA’s research animals should experience a minimal amount of pain and distress. Animals should only be sent into space when an alternative experiment cannot be conducted on Earth, as few animals as possible should be sent, and ‘lower’ life forms, such as insects, should always be used when appropriate.[80]

These may be self-imposed rules, but NASA is also required to abide by the United States Department of Agriculture Animal Welfare Act, and the Public Health Services Policy Act, which sets minimum standards for the care of research animals. Not every country has laws to protect animals used in experiments but it is hoped that international projects, like the ISS, will provide a reason to develop international standards.[80]

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