26. Life on Earth

Transcript: The history of life on Earth is not a simple linear progression from simple to more complex, from bacteria to us. There have been many twists and turns in this tale, many evolutionary dead ends. Chance effects are important on the history of life, for example, the role of giant impactors from space causing mass extinctions. The arrival rate of these huge impactors is utterly random and cannot be predicted. Chance also plays a role in the microscopic process of mutation which leaves the diversification of genetic material. As the physician Lewis Thomas has said, “The capacity to blunder slightly is the real marvel of DNA. Without this special attribute we would still be anaerobic bacteria, and there would be no music.”

Transcript: If planetary scientists are asked to speculate on the most possible sites for life within the solar system, they will generally give five places: Mars, Venus, Europa, Titan, and Io. These five places are significant. Mars is a traditional place where we might imagine life could have existed, and Venus is our sister planet in many ways. However the other three sites are moons in the outer solar system, beyond the habitable zone. Their selection as possible sites for life indicates two things, first, that life can get its energy from a variety of mechanisms. Direct sunlight from a star is not the only possibility. Geothermal energy or tidal heating are also possible. Second, we are informed by the existence of extremophiles on Earth that can subsist under a wide range of physical conditions that the range of conditions in the solar system that could harbor biochemical life is also quite large, so we should be generous in including sites for life where conditions might be quite extreme.

Transcript: It’s particularly important to consider the possibility of life on or in the gas giant planets of the solar system because these are the type of planets that have been found around nearby stars. Over a hundred extrasolar planets are now known, and most of them are Jupiter-like or larger, although they are on tight orbits of their parent stars and are at higher temperatures. It will take a decade or more before Earth-like extrasolar planets are found in significant numbers. The gas giant planets are unlikely environments for life. They are just like the Sun in chemical composition with primarily hydrogen and helium. Even if they have rocky cores, the conditions close to the rocky core will be extremes of temperature and pressure that make it implausible for biological life to survive. There has been speculation that in the atmospheric regions of these gas giant planets, a moderate temperature is reached at some elevations, say, a hundred kilometers into Jupiter’s atmosphere. Carl Sagan speculated about life forms circulating in the atmospheric regions of Jupiter. However, the vertical currents of air in Jupiter’s atmosphere are extreme and it’s almost implausible that any buoyant organism could exist and be stable for a significant length of time. Thus, most astronomers think that life on or in the giant planets of the solar system in very unlikely.

Transcript: Given trillions of potential sites for life and billions of years for it to evolve, it’s impossible for scientists to know how strange life might be or how different from life on our planet. In the discussion of life in the universe, scientists tend to make the strong assumption that life elsewhere will be biologically based. We know that DNA is not the only replicating molecule, but we certainly assume that carbon-based chemistry is at the core of life on other planets around other stars. But what if it’s not the case? What if life could form deep in an interstellar cloud? It seems unlikely because the density is so low. What if gravity or electromagnetic fields could somehow self-organize and produce complexity and store information? It may be possible, but we have no mechanism at the moment. Another possibility is that biology is just one phase of evolution. Perhaps a simple extrapolation of our own experience with technology and biology is that within a few generations, the organism and machine will merge, and perhaps the frailty of a biological organism will eventually be replaced by machine-like intelligence. Artificial life could have so many forms, it’s hard for us to imagine, but scientifically we must consider the possibility that life beyond Earth might be non-biological.

Transcript: The first place to conduct the search for life beyond Earth is our own solar system. Each of the planets in the solar system is hundreds of thousands of times closer than the next nearest extrasolar planets around nearby stars. So astronomers must start by closely inspecting all the planets and the major moons of the solar system for evidence of life. We can use remote sensing techniques, spectroscopy to tell us what the chemical composition of the surface material is. We can use flybys to measure in different regions of the electromagnetic spectrum, and in some cases we can use landers. We have to remember that evolution is an issue. The solar system conditions may have changed over billions of years. The habitable zone, the region of liquid water, that now encompasses only the Earth, may once have included either Mars or Venus. We may start by ruling out some environments. Mercury is almost certainly too hot and has almost no atmosphere. It's a moon-like object and so is unlikely to have life. Pluto and small bodies in the outer solar system are also unlikely to have life because their liquid materials and even their organic materials have been frozen. Asteroids and comets have ices, and they have been liquefied at times for never more than a million years at a time which most biologists think is too short of for life to develop. So we can start by ruling out some of the more extreme environments in the solar system.

Transcript: Computers have informed us about evolution in two fundamental ways. First, the power of computers has been used to model evolution. It’s possible to create life in a computer in a sense. So-called genetic algorithms can be used to mimic the processes by which life may have begun on Earth. This is of course not a true simulation of wet or biochemical life, but it's a computer representation. The insight from these models is substantial though because it shows that with simple ingredients and interactions complexity, self-organization, and highly complex networks of activity can naturally evolve. These might be chemicals, but they also might be the precursors for intelligence if such a thing happened in an electrical network, like a brain. The second way that computers have informed us about evolution is of course in their ability to mimic the high level human functions of the brain. Artificial intelligence to many has been a disappointment in the last few decades because there is no computer that can think like a person. Many people were shocked when a computer finally beat the chess master of the world. However, in a sense this was a simple trick of pure computational power. Computers cannot yet mimic the complex brain function, but a projection of Moore’s law shows that within a few decades they might. And at that point we will have the first signs of whether artificial life is possible.

Transcript: Venus is in many ways the sister planet to the Earth. However, the physical conditions are so extreme and unpleasant that it’s an irony that it’s named after the goddess of love. Venus has extremely high atmospheric pressure, ninety bars, which is the s [transcript incomplete-we will complete shortly 7-29-11]

Transcript: We tend to associate technology with our high intelligence and our role as masters of this planet, but it’s worth bearing in mind that it’s possible to have intelligence without technology. Dolphins, orcas, and other marine mammals may be highly intelligent, but they do not have opposable thumbs and they will never build telescopes and wonder about their place in the universe. In terms of evolution of technology, we have accelerated the rate of complexity in life. Early life’s DNA only increased in its information storage capacity at a rate of about one bit per hundred years. That rate accelerated during the time of human DNA a hundred times to about one bit per year. Now with modern computers, technology, and information technology, we are creating a megabit per second on this planet. So we have accelerated the rate of information storage and information capacity on this planet enormously. We are obviously living in a time of exponential increase of information capacity. Whether this allows us to transcend our biological evolution, we do not know.

Transcript: We tend to think of tools and technologies in terms of human development and our control over the natural environment, but life has more generally evolved technologies to solve the problems of adaptation and evolution. Examples include flight, which was invented 150 million years before humans invented it, the detection of infrared waves, which is done by snakes and some forms of fish, sonar detection, done by bats, the sensing of magnetic fields, done by birds and also some fish, and communication by pheromones, or chemical signals, which occur in many species at a rate of more than 100 bits per second, equivalent to about 20 words per second in the English language. These examples of technology may not seem as impressive as our control of the natural environment, but, given sufficient time, it’s unclear how diverse the technologies evolved by life on other planets might be.

Transcript: Venus is in many ways the sister planet to the Earth. However, the physical conditions are so extreme and unpleasant that it’s an irony that it’s named after the goddess of love. Venus has extremely high atmospheric pressure, ninety bars, which is the s [transcript incomplete-we will complete shortly 7-29-11]

Transcript: Imagine the Earth sixty million years ago, not long after the death of the giant reptiles and dinosaurs. On the plains of Africa, descendants of mammals became apes and moved into the trees. They had complex skills of hunting and manipulating tools. They had binocular vision. They had large brains, and they cared for their young and socialized their young. They lived in tribes. They had a complex language. These were our distant ancestors. The ancestral primate about sixty million years ago split off the chain that would become lemurs. About forty million years ago, old and new world monkeys split off the genetic tree. About thirty-five million years ago, gibbons, about fifteen million years ago, orangutans, and only seven or eight million years ago, gorillas and chimpanzees. Contrary to popular perception, humans did not evolve from apes or gorillas, but we all share a common ancestor.

Transcript: Humans are special. There is no escaping that fact. On this planet, humans are the only creatures that have evolved the capability to adapt to their environment and control their global environment. Humans have also developed the ability for abstract thought, for mathematics, and humans have figured out ways of understanding the entirety of the universe that they live in which is a fantastic achievement in only a few thousand years. But what at a genetic level is special about the large brains of humans? In one sense, the complexity is truly amazing. The human genome has information content in a four letter alphabet equivalent to a large book or an encyclopedia, some billions of bits of information. However, the human brain has ten to the twelve or a trillion cells, and these cells have ten to the power fifteen or a thousand trillion connections. These electrical-chemical connections and the networks they form are the basis for our intelligence. Ten to the power fifteen is a huge number, and it implies a large amount of complexity. Obeying Moore’s Law, if computers were to continue to develop as they have done, computers could potentially rival this level of connectivity and storage of information in about twenty years. We have no idea whether this means that humans can develop technology that becomes intelligent as well.

Transcript: The normal rate of evolution via natural selection is slow at a genetic level. There typically is one mutation for every 100,000 divisions of a cell. However, when brains form, the acceleration of evolution is possible, because brains enable the manipulation of the environment. On Earth, brain size tends to scale with the size of the organism. Humans have a relatively large brain for their size, but similar ratios are seen in orcas and other marine mammals, and elephants are not that dissimilar. We believe the complexity of the human brain outstrips the complexity of any other brain of an animal on Earth, but evaluating the intelligence of other animals that we share the planet with is difficult. Orcas, for example, are top of the food chain in the oceans, have a large and complex brain surface, have a complex language, as yet not understood by us, engage in play, have complex social networks, hunt in groups, and do other sophisticated behaviors. How intelligent are orcas, or even other great apes? It is hard to say. We share 95% or our DNA with the great apes, and some species of chimpanzees are so similar to us that they’re barely a different species, and yet humans are special in their high capacity for abstract thought.

Transcript: When we think about life in the universe, we tend not to think of microbial life clinging to a planet around a nearby star. We tend to think in terms of intelligent life. But in terms of talking about the probability of intelligence elsewhere in the universe, we have to understand the role of intelligence in the evolution of life on Earth. Large brains with the capability for intelligence probably evolved as a natural consequence of natural selection. Brains would have conveyed an adaptive advantage on complex creatures that had brains, because these nerve centers for the organism would have enabled superior hunting strategies and strategies with dealing with the climate with its variations, and predators. However, it’s not obvious that large brains with the capability for abstract thought are an inevitable consequence of evolution. For example, we share the planet with blue-green algae, and simple microbial organisms that have lived for billions of years, and in essence are far more successful life forms than humans are. Also, if humans or other intelligent species disappeared from the planet, which is the fate of almost all species, there is no guarantee, and it’s not obvious, that intelligence would once again arise.

Transcript: Humans have existed only for a couple of million years, a tiny fraction of the span of life on Earth. For most of this time, we left a small footprint on the Earth. Humans were just hunter-gatherers, and there were only a few million or tens of millions of them until relatively recently. Now there are over six billion humans, and in the last few hundred years and at an accelerating rate in the last fifty years, we’ve been altering the global chemistry and climate of our planet. There is demonstrable impact of industrial activity on the carbon dioxide level in the atmosphere and, arguably, global warming itself. Also, our use of the planet as a consumable resource has lead to a stunning loss in biodiversity in the last few decades. Looking backward in time, this period will be viewed as a great dying on par with any of the mass extinctions in the history of the Earth. Apollo 8 gave us a view of our planet as a delicate orb suspended in space. Only time will tell whether we can tread lightly enough on our planet to leave it for our distant ancestors.

Transcript: Mass extinctions cannot only be caused by impacts from space or by violent geological change. They can also potentially be caused by the death of stars. Supernova represents the death of a massive star, leaving a compact remnant of a neutron star or black hole. When a supernova goes off, there is a huge amount of light emitted, also a large amount of high energy particles or cosmic rays. The incidence rate of supernova is very low, only one in the entire Milky Way galaxy every hundred years. It’s been four hundred years since humans witnessed supernovae, and the nearest versions four hundred years ago were several hundred lightyears away. If however, a massive star went off or died within ten or twenty lightyears of the Earth, it would release a flood of cosmic rays that would cause a spike in the mutation rate of species and probably could lead to a mass extinction. We don't know when or if such an event happened, but statistically it is likely that over the history of life on Earth, several times, dying stars have gone off within a few lightyears of the Sun.

Transcript: Impacts from space have been implicated in several of the mass extinctions in the last half billion years. However, they are not the only possible cause. Geological change can profoundly affect life, particularly during periods of intense volcanism. Also, climate fluctuations can impact the survival of life, especially during times when the temperature non-linearly oscillated and led to extremes of heat and cold. There were times, for example, when the Earth was probably frozen down to its equator, or impacts could be responsible. On average, once every million years a one kilometer size impactor hits the Earth, devastating a continent sized region and causing global climate change. Once every hundred million years, a ten kilometer sized impactor can cause a mass extinction and a global catastrophe. How can we tell? The two key pieces of evidence are that the extinction occurs in an instantaneous interval as well as can be measured by geological techniques, but the fossil record is only accurate to about a hundred thousand years which is not the same as saying that the event was instantaneous. The second critical piece of evidence is a smoking gun, evidence of a crater of appropriate size that’s dated back to the same time as the extinction occurred.

Transcript: The best evidence we have that mass extinction can be caused by an impact was the extinction that occurred sixty-five million years ago at the boundary between the Cretaceous and Tertiary periods, the so called KT impact. An impact directly killed forty percent of all plants and animals and ultimately through its effect on the atmosphere and global cooling, seventy-five percent of all plants and animals. Possibly ninety-nine percent of all the organisms were killed in this event. Among the few survivors were mammals, which of course were our descendants. The evidence that this mass extinction was caused by an impact is very good. There is crater of exactly the right age off the Yucatan Peninsula. The fossil record shows and almost instantaneous loss of species, and there is widespread or global evidence for shocked quartz, for soot layers corresponding to fires that were initiated, and for deposition of iridium which is material that only comes from outer solar system material.

Transcript: Species have been developing and becoming extinct throughout the history of life on Earth. Extinction is not unusual. However, there have been certain periods of time when the extinction rate increases dramatically. Essentially large fractions of the number of plant and animal species on the entire planet become extinct within a time span that is geologically short. There have been five major mass extinctions in the history of the last half billion years. Before this time there simply wasn't a good enough fossil record to know what happened in terms of the extinction rate. At the late Ordovician period, at the beginning of the Silurian, four hundred and twenty million years ago, about fifty percent of all species became extinct. Then again, about three hundred and eighty million years ago in the late Devonian period, about twenty-five percent of all species became extinct. Large extinction of half the species occurred at the end of the Permian era about two hundred and forty million years ago and then again relatively shortly afterwards about two hundred and twenty million years ago at the end of the Triassic and the beginning of the Jurassic. Perhaps the most famous mass extinction occurred sixty-five million years ago at the boundary between the Cretaceous and the Tertiary period when the dinosaurs and many other species of plants and animals became extinct in a relatively short period of time.

Transcript: The idea of contingency was invented by Alfred Russell Wallace, a rival to Charles Darwin and co-discoverer of the idea of natural selection. In contingency, life diversifies according to genetic principles and then is pruned back according to adaptation to the natural environment, but the pruning or culling of life has an element of chance, of a lottery. Paleontologist Steven J. Gould, in his book Wonderful Life, wrote about the diversification of life in the oceans of the Earth during the Cambrian explosion. He argued that it was impossible with hindsight to decide which of the many body plans invented during this period would have or could have survived according to natural selection. The element of chance was strong. He went further and argued that if you replayed the tape of life on Earth under identical conditions, it would not be obvious that eventually you would get mammals or apes or Homo sapiens. We can see contingency in the Cambrian survivors and also in the more famous incident where the mammals and the dinosaurs coexisted. Rather than thinking of the mammals as natural and obvious successors to the more primitive dinosaurs, we should realize that mammals and dinosaurs coexisted successfully for over a hundred million years before the dinosaurs were wiped out by a chance event, the impactor from space.

Transcript: Life on Earth originated in water, and it grew to multicelled complexity in the oceans. Small organisms, however, could have found plenty of niches on land to survive, in ponds or in small pools of water. It’s likely that algae did this over half a billion years ago. Larger organisms must have taken longer to reach the land because they needed a way of gathering nutrients from the soil and the air rather just from water. Plants and fungi were probably the first to make the leap to land, facilitated the by the rise in oxygen and the ozone layer which would have protected organisms on land from cellular damage. Four hundred and fifty million years ago, perhaps four hundred and eighty million years ago, the first plants reached the land. Animals followed about four hundred million years ago. The first animals on land were of course amphibians, able to survive both a liquid and a dry environment. By three hundred and fifty million years ago, the Carboniferous era, vast forests filled with insects populated the land mass, and of course over this period the deposition of organisms in layers of sediment lead to the deposition of coal and oil that fueled our industrial revolution.

Transcript: Half a billion years ago, Earth’s oceans present a perfect environment for the diversification of life. It was a ready source of nutrients and a long term stable environment for diversification and natural selection to operate. In the Cambrian period, the fauna included the first hard bodied sea creature, the trilobite, which lived for two hundred million years but did not survive to the present day. Many other body plans were laid down in that era that did not survive to the present day. Natural selection acts to cull according to the success rate of an organism, it’s ability to adapt and reproduce. It’s sobering to realize that 99.999 percent of all species in the history of Earth have become extinct. Even successful creatures that have survived for hundreds of millions of years eventually become extinct. Our place in the history of life is not yet secure.

Transcript: The development of multicellularity was very important in the evolution of life, and it spurred the adaptive process of natural selection. There is evidence that multicellular organisms developed independently in several branches of the eukaryotes. However, about five hundred and fifty million years ago, in the oceans of the Earth, something extraordinary happened. This was the start of the Cambrian era, and it dates to a time when there was an extraordinary flowering of life forms in the oceans of the Earth. The diversity in the Earth’s oceans lead to almost all the major body plans of life that we see now, occurring only in a one percent span of the Earth’s history starting five hundred and fifty million years ago. Biologists talk about the body plans of organisms by grouping them in phyla. The level above this are kingdoms, such as the plant kingdom and the animal kingdom. The Cambrian explosion saw the development in a short period of time of thirty or more phyla, including for example the arthropods which includes insects and spiders and the chordates which include mammals and reptiles. This flowering of life in the oceans of the Earth was facilitated by the rise in oxygen in the Earth’s atmosphere and probably by the fact that there were not a lot of predators at the time of the first diversification.

Transcript: About six hundred million years ago in the oceans of the Earth the first multicelled organisms developed. These are distinct from colonies of simple cells like stromatolytes which existed much earlier. A multicelled organism is a large number of cells acting in concert within a single creature. In the evolution of life, the transition from prokaryotes to eukaryotes, cells with nuclei, spurred a huge amount of biological diversity. The transition from single-celled nuclei, eukaryotes, to multicelled organisms spurred an even larger amount of diversification. The diverse functions of cells led to specializations of their function. The cells were independent, and the organism was able to adapt better to the environment. The ceaseless experimentation of multicelled organisms lead to new capabilities. As just one example, consider the cells that evolved on the surface of a creature that were able to detect light. Eventually those cells could aggregate and form, over hundreds of millions of years, eyes.

Transcript: Simple organisms such as bacteria reproduce by making copies of themselves. Once cells had developed the capability of having nuclei, sexual reproduction became possible, and this is very important for genetic diversification. In sexual reproduction, the offspring get half their genetic material from each of the parent organisms. Genes can combine in different ways from generation to generation which facilitates experimentation and adaptation to a changing environment.

Transcript: Ice ages are periods of global cooling by a few degrees, up to ten or so degrees, that occur at irregular intervals of tens of thousands to hundreds of thousands of years. The causes of ice ages are complex, but over the past few million years, there is good evidence that variations in the Earth’s orbit has contributed to the cause of ice ages, in particular the Earth’s tilt on it’s axis, which is varied from twenty-two to twenty-five degrees, influenced primarily by Jupiter. Other influences in the ice age could be the geomagnetic reversals that occur every few hundred thousand years in the Earth’s core which are also influenced by orbital dynamics. Astronomers are only beginning to unravel the effects of subtle changes in the Earth’s orbit and their impact on global climate.

Transcript: There is over 100,000 times more carbon dioxide locked up in the ocean and the rocks of Earth than there is in the atmosphere. If even a tiny percentage of this carbon dioxide were released into the atmosphere, it would lead to a runaway greenhouse effect that would raise the temperature of the Earth to the level Venus and beyond, and make life extremely difficult. Thus, there appears to be a fundamental connection between the regulation of carbon dioxide and the carbon dioxide cycle, and the stability of the Earth’s atmosphere long enough to allow complex life to evolve, and the key aspect of this is the presence of plate tectonics, because plate tectonics leads to the subduction of CO2-bearing rocks, and the eventual release of the gas back into the atmosphere through volcanism; thus, it’s a key part in the carbon dioxide cycle. We do not know if planets elsewhere in the solar system or in the universe must have plate tectonics to have well-regulated atmospheres, but appears to be no coincidence that on Earth, plate tectonics help to make the Earth a habitable place.

Transcript: In addition to gradual change over billions of years, Earth’s climates has been fluctuating and subject to instabilities that have taken it to extremes that are hard for us to imagine. In a period of six hundred to seven hundred and fifty million years ago, Earth was subject to a series of deep ice ages when glaciers reached nearly to the equator, and the oceans froze to a depth of about a kilometer. This is hard for us to imagine. It got this way because a normal fluctuation made the Earth cooler at which point the glaciers advanced, and the oceans began to freeze. Ice reflects ninety percent of light incoming rather than five percent for water. As the oceans reflect more light, they get colder still accelerating the freezing process in a runaway fashion. However, during this time volcanic activity, driven by energy sources deep in the Earth, does not diminish, and so eventually carbon dioxide is released which is not absorbed in the oceans because they are frozen. So it builds up in the atmosphere, warming the planet and melting the ice, and more sunlight gets absorbed. This also becomes a runaway process, and the large amount of carbon dioxide leads to an overshoot, an enormous warming of the temperatures. Thus, there are instabilities in the Earth’s atmosphere that can lead to climate extremes which must have affected life on Earth at the time.

Transcript: The stability of the Earth’s atmosphere over billions of years has been impacted in part by the carbon dioxide cycle which has acted as a thermostat to regulate the temperature of the Earth, even over time spans when the Sun was changing its brightness or the amount of tectonic activity was varying. The basic chemical principle is that the rate at which carbonates form depend on temperature, and carbonates from faster when it’s hotter. Thus when the temperature increases, the rate of carbonate formation increases which means the rate at which they dissolve in the oceans increase, and the dissolution of carbon dioxide in the oceans removes it from the atmosphere thereby lowering the greenhouse effect which causes the temperature to fall. On the other hand, if the temperature of the Earth is reduced for any reason, the rate of carbonate formation also falls as does the rate of its dissolution in ocean water, which leads to more greenhouse gas, which leads to a higher temperature. Thus, the process is self-regulating.