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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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....

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...

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...

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...

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...

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...

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...

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...

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...

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,...

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.