CHAPTER 6 THE ORIGIN AND EVOLUTION OF LIFE ON EARTH
This chapter kind of talks about how life most likely have occurred.
We learn these things from geological records. It is full of surprises.
6.1 Searching for Life's Origins
We don't know, from geological record, or biological evolution, how life arose on Earth precisely. The geological record
becomes increasingly incomplete as we look further back in time, and no known rocks survive from Earth’s first half
billion years.The theory of evolution explains how one species can evolve into others, but does not tell us how the first
living organisms came to be. So basically we study how life originated from limited clues and laboratory experiments.
When did life begin?
We study how life occurred through on approach called fossils. We find a fossil of a particular age, then we know life
already exited at that time. For example, a 3.5billionyearold fossil tells us that life on Earth arose before 3.5 billion
years ago.We don't have a complete record of fossils and so we don't have the oldest one to figure out how long life has
existed on Earth.
Currently, three likes of fossil evidence all point to the idea that geologically speaking, life arose quite early in Earth's
history. The first life of evidence comes from stromatolites which are rocks that are characterized by a distinctive layered
They are identical to sections of mats formed today by colonies of microbes sometimes called living stromatolites.
Living stromatolite contain layers of sediment intermixed with different types of microbes.
Microbes near the top generate energy through photosynthetic microbes.
Then the ones beneath use organic compounds left as waste products.
The living stromatolites grow in size as sediments are deposited over them, forcing the microbes to migrate upward in
order to remain at the depths to which they are adapted.
This gradual migration creates the layered structures.
The similar structure between the two suggests a similar origin, implying that stromatolites are fossil remnants of early life.
There is controversy because geological processes of sedimentation can mimic their layering.
However, the wide variety of structures seen in stromatolites and the results from chemical analysis make most scientists
confident that they offer evidence for the existence of microbial colonies as far back as 3.5 billion years ago.
Another evidence: If the old stromatolites are like the living mats today, it means that photosynthesis is a process that
existed 3.5 billion years ago.
Since photosynthesis is such a complex process that must have took a long time to evolve in living organisms, it means that
primitive life existed even earlier.
The second line of evidence of the first life on Earth comes from individual fossilized cells. This is called microfossils.
This is hard because rocks become increasingly rare with age and because the oldest rocks have been altered by geological
processes in ways that tend to destroy microfossils within them. Therefore they're really hard to find.
When we find an interesting bone fossil, it's hard to determine if it's biological or mineral in origin. So it's hard to explain
their origin and therefore creates controversy.
The greatest controversy surrounds the oldest claimed microfossils, which come from a 3.5billionyearold rock formation
in northwestern Australia.
At the beginning scientists said they represented fossils of early photosynthetic organisms, because it had chemicals
structures that belonged to rocks from the shallow sea.
However, subsequent analysis has shown that the rock is not from a shallow sea as originally assumed, but instead must
have formed near a deepsea volcanic vent similar to a black smoker.
Therefore they cannot be fossils of photosynthetic microbes, and therefore they are not fossils at all but just some stuff
formed from nonbiological process.
Another possibility is that while these structures revealed the rock to be 3.5 billion years old, they may have come to this
location from recent flowing water There are some less sketchy evidence, from 2 locations in southern Africa where the rocks are dated to be 3.2 and 3.5
billion years old. They are more believable.
These rocks and the ones on australia appear to share a common history, so they were geologically linked in the past, so
this makes it more possible that the ones in Australia is also real.
Anyways, the rocks that are 2.7 billion and 3.0 billion years old show more certain evidences for life.
Overall, we conclude that microfossil evidence clearly points to the existence of life before about 3.0 billion years ago and
may well tell us that life existed before 3.5 billion years ago.
The third line of evidence for an early origin of life comes from isotopic analysis of some of the most ancient rocks on
Carbon has two stable isotopes: carbon12, and carbon 13
Carbon 12 has 6 protons and 6 neutrons in its nucleus, and carbon 13 has 6 protons and 7 neutrons.
Carbon12 is more common but any inorganic carbon sample always contains a small proportion of carbon 13 mixed in
there. (Like 1/89 carbon atoms)
When living organisms metabolize carbon, they incorporate carbon12 atoms into cellular molecules slightly more easily
than they do carbon13 atoms.
Therefore, living organisms and their fossils have lower proportion of carbon 13 atoms than inorganic materials.
This kind of proportion was found on an island off the coast of Greenland, on rocks that are 3.85 billion years old. This
suggests that life existed at that time.
This was challenged by a few scientists because the rocks are metamorphic, so they may be transformed substantially by
heat, which explains why no intact microfossils remain within them.
Other rocks dating 2.8 billion years ago are found in the area so it's likely that these rocks are legit evidence.
In addition, not only do they have proper carbon ratio of life, but they also have the proper ratio of other elements such as
iron, nitrogen and sulfur.
So with the above reasoning, if we are interpreting all the data correctly, it is likely that life arose considerably earlier than
3.85 billion years ago. Geologically speaking, this would mean life arose quite early in Earth’s history
It suggests that we could expect life to also arise rapidly on any other world with similar conditions.
What did early life look like?
Some living organisms went extinct long ago, but others coexisted with dinosaurs such as sharks and alligators.
Studying these primitive species helps us to understand what early life looked like, which in turn should help us investigate
the question of how life arose.
Because living species have evolved from common ancestors, the base sequence in the DNA of living organisms provides
a sort of map of the genetic changes that have occurred through time.
By comparing the genomes of different organisms, we should be able to reconstruct the evolutionary history of much of
life on Earth.
Mutations created variations on a DNA, and each new species therefore had a slightly different DNA from the oldest
However these changes are traceable and after determining the sequence we can trace back to the base sequence of living
Two species with very similar DNA sequences probably diverged relatively recently in evolutionary history, while two
species with very different DNA sequences probably diverged much longer ago.
The tree of life is mapped out with these DNA sequences.
This knowledge is what tells us that life can be divided into 3 domains, bacteria, archaea, and eukarya.
Species near the root of the tree must be more similar than other species to the common ancestor of life on Earth.
Where did life begin?
To answer this question we rely on ideas about geological knowledge.
It's unlikely that life arose first on land.
There wasn't an ozone layer to protect the plants.
So the more hospitable environments are underwater because water absorbs UV light and under rocks. One such possibility, first suggested by Darwin, is shallow ponds.
Organic compounds may have formed spontaneously in such ponds.
Once the compounds formed, tides or evaporation could have increased their concentration near the pond edges, spurring
reactions that might have led to life.
Volcanic hot springs may also have offered energy to support an origin of life. However, while these factors suggest ponds
could have been a good location for an origin of life, the shallow water would not have offered much protection against
solar ultraviolet radiation. Even if they did form, heavy bombardment would have destroyed everything except the life in
deep sea or underground.
Deep sea is a better possibility because there's more protection from UV lights.
Deepsea volcanic vents offer plenty of chemical energy to fuel reactions that might have led to life, and chemical energy
is also available underground in reactions between water and minerals in rock.
For that reason, it now seems likely that the common ancestor of all life on Earth today evolved from organisms that lived
near deepsea vents or underground, even if the first origin of life occurred elsewhere.
6.2 The Origin of Life
Lab experiments try to recreate the chemical conditions that should have prevailed on the early Earth,which is an
assumption that life originated here.
However, it's possible that life migrated to Earth from another world.
It's also possible that life arose through some divine intervention, that falls on a matter of personal faith.
How did life begin?
Life is baed on chemistry of organic molecules.
Therefore we assume that life was produced from chemical reactions on the early Earth.
This isn't a natural process because Earth's oxygen atmosphere prevents complex organic molecules from forming living
The oxygen in our atmosphere is so reactive that it will attack chemical bonds and remove electrons and stuff, and
eventually destroy organic molecules.
However, the oxygen in our atmosphere is a product of life, produced by photosynthesis, which means it can't be around
before life existed.
Therefore, young Earth must have been like a giant lab for organic chemistry.
Some scientists thought Earth's atmosphere should bee oxygen free in the beginning.
They thought the creation of organic molecules was this spontaneous thing, and that it came from sunlight fuelled
This idea was put to the test by Stanley Miller and Harold Urey, so it is known as the Muller Urey experiment.
Earth was represented by a small glass flask with water. The atmosphere was represented by water vapours.
They added gaseous methane and ammonia in the water vapour to represent the atmosphere.
There's a central flask that supplied energy, basically it was electrical charges.
What happened was the vapour travelled to the flask below and cooled, and then it condensed and flowed back into the
The water which was supposed to be ocean turned murky brown, and after a chemical analysis we found out that it
contained many amino acids and other organic molecules.
The methane and ammonia mixture in the experiment wasn't representative of Earth's early atmosphere, so the experiment
was pretty much pointless.
At least it proved that under some conditions the building blocks of life form naturally and abundantly.
Scientists tried to change the ingredients in the experiment for a better answer, but nope it was all shitty.
Although we don't know much about how much organic materials might have been made, we still know two other sources
The first of these sources is chemical reactions near deep sea vents. Under sea volcanoes heat the surrounding water and a
variety of chemical reactions can occur between water and minerals.
Another source of organic molecules may be material from space. Meteorites often contain organic molecules, including
The Stardust mission collected dusts and comets and brought them back to Earth. We study them and apparently organic molecules can form under the conditions presented in these materials and can survive the plunge to Earth.
Natural ultraviolet light from the young Sun could also have produced some of the building blocks of life.
Chemical reactions will occur on dust grains orbiting the Sun and then these dust with organic molecules might have
rained down on the young Earth, maybe it came from the heavy bombardment.
So yea, these sources must have together shaped the chemistry of early Earth. With three sources, its likely that at least
parts of the organic molecules became the basis of life.
Now that we have the building blocks of life, the next question is how they might have assembled into a living cell.
One way to explore how this transition happened was to work backward from organisms living now.
DNA has the ability to replicate and shape heredity. Early life must have had something too, but probably not DNA, since
it's too complex.
It has like double strands and intertwined with RNA and proteins, it can't bee the genetic material of the first living
So here we're talking about some kind of molecule that is simpler than the DNA but still capable of making accurate
copies of itself, RNA.
RNA is simpler than DNA because it only has one strand than 2 and it's structure requires very few steps in its
The problem with this idea was that both DNA and RNA, in today's world, can only replicate with the help of enzymes.
These enzymes are proteins that are made from genetic instructions contained in DNA and carried out with the help of
RNA.RNA cannot replicate without enzymes, and the enzymes cannot be made without RNA.
This problem is solved by Thomas Cech in 1980. They found that RNA can catalyze biochemical reactions in much the
same way as enzymes.
We now know that RNA molecules play this type of catalytic role in many cellular functions, and we call such RNA
These discoveries have led biologists to envision that modern, DNAbased life may have arisen from an earlier RNA
world, in which RNA molecules served both as genes and as chemical catalysts for copying and expressing those genes.
How did an RNA world start though? Well first there's the spontaneous production of self replicating strands of RNA.
There's not enough organic molecules for RNA to just spontaneously reproduce. This would have required some catalytic
reaction to happen.
There is a few types of inorganic minerals that can self assemble complex organic molecules, they're called clay, and
It's common in the world, in the oceans because it forms from silicate minerals. We have an abundance of it.
Clay has this layer of molecules that other molecules stick to. When this happens, the mineral surface structure force them
into such close proximity that they react with one another and form longer chains.
This easily produces RNA strands, but are too short to produce self replicating RNA.
It's only 12 base long, but we need them to be 165 bases long.
Problem with clay: The RNA strands are weakly bonded, and peel away easily.
But apparently even RNA 5 bases long can act as a ribozyme. So simple ribozymes catalyzed folded RNA molecules and
made it longer and more complex.
So this means it's really likely that RNA molecule were capable of self replication.
Other experiments show that RNA with other organic molecules could easily become confined by things things called pre
Pre cells form in 2 ways. One is by coolling a warm water solution of amino acids so they form bonds among themselves
to make an enclosed spherical structure. Or they form by mixing lipids with water.
They exhibit some important properties of the membranes of living cells.
They do some crazy living cell shit like storing energy and allowing certain molecules in and out. They can also keep
growing and then split into "daughter spheres." Experiments show that they also form on the surface of clay minerals.
Storing RNA and other organic molecules in pre cells can facilitate the origin of life because of 2 things.
First, keeping molecules concentrated and close together should have increased the rate of reactions among them, making
it far more likely that a selfreplicating RNA would have arisen.
Second, once selfreplicating RNA molecules came to exist, precells would have kept them isolated from the outside, this
way RNA molecules that replicated faster and more accurately would rapidly come to dominate the population.
If the enzyme floated freely within the ocean water, it might just as easily have helped the replication of other RNA
molecules as of the one that made it. But inside a precell, the enzyme would help only the RNA that made it.
At some point, the RNA pre cells would likely have become sufficiently good at reproducing and evolving to be “alive.”
This is probably some gradual thing so there is no moment that the first living cell appeared on Earth.
Once the first living organisms of RNA world arose, biological natural selection could over.
Eventually DNA will evolve within living cells.
Because DNA is a more flexible hereditary material and is less prone to copying errors than RNA, life that used DNA for
its genome would quickly have outcompeted the remaining organisms that used RNA.
But RNA served many other cell functions well, explaining why RNA still plays so many important roles in cells, even
though it no longer plays a hereditary role.
Summarize these points:
1. Naturally forming organic molecules are the building blocks of life.
2. Clay minerals catalyze production of RNA and membranes that form pre cells
3. Molecular natural selection favours efficient, self replicating RNA molecules.
4. True living cells with RNA genome give rise to the RNA world.
5. DNA evolves from RNA and biological evolution.
Could life have migrated to Earth?
The idea that life might have traveled through space and landed on earth is called panspermia.
The presence of organic molecules in meteorites and comets tells us that the building blocks of life can survive in the
Meteorites gets blasted off the Moon and Mars and eventually land on Earth.
In a sense, Earth, Venus, and Mars have been “sneezing” on each other for billions of years, offering the possibility of
microscopic life hitchhiking between worlds on one of the meteorites.
For life to survive through this journey involves overcoming 3 obstacles.
The impact that blasts it off the surface of its home world, the time it spends in the harsh environment of interplanetary
space, and the fiery plunge through our atmosphere.
Only the middle obstacle is a bit difficult to overcome.
The chance of surviving the trip between planets probably depends on how long the meteorite spends in space.
When things get launched into space they will orbit the Sun, and eventually it comes across the path with Earth.
This might take millions of years and so during this time it's unlikely that life will survive such a long time. Only the ones
that comes to earth very quickly can bring life here, which is like 1/10 000, that takes less than a decade.
Life is often killed in this space traveling process because it is exposed to cosmic rays, or from desiccation, the lack of
Today, most ideas about migrating life fall into one of two broad categories.
The first broad idea suggests that life was migration from elsewhere. They said that life can't form as easily as we think, esp. with the conditions present on the early Earth.
This theory allows the possibility that another world has conditions that can develop life.
Problem with this idea is that other worlds in our solar system should have had no more time available than Earth, and why
would any of these worlds would have offered better conditions for an origin of life.
The second broad idea suggests that life forms so easily that we should expect to find life originating on any planet with
In this case, since Venus and Mars were originally very suitable for life, life should have arose in those planets first.
Since we blow our nose at each other, then the microbes on Earth could have traveled to other places as well.
They traveled to Mercury, the Moon, Mars and Venus. But they didn't survive in anyplace. Actually they might have
survived on mars temporarily.
Problem is: If we find life on Mars then we don't know if it's native or came from Earth.
Second: If life can migrate among planets like this, can we even distinguish between indigenous origin of life on Earth
and origin based on migration?
One way or another, life arose on Earth quite soon after conditions first allowed it, and even if life migrated here from
another world, we have good reason to think that it evolved naturally, through chemical processes that favor the creation of
complex, organic molecules and the subsequent molecular evolution of selfreplicating molecules.
6.3 The Evolution of Life
Here we will retrace the history of life on Earth as it is currently understood, which will help us understand the
possibilities for finding similarly complex life on other worlds.
What major events have marked evolutionary history?
We find a few key events that marked the evolution and help us understand stuff.
Even though the early organisms were simple, they still had a few enzymes and a rudimentary metabolism.
Probably looked like modern bacteria or archaea. Lacked cell nuclei and other complex structures we find in eukarya.
Because the atmosphere at that time was essentially oxygenfree, all early life must have been anaerobic, meaning that it
did not require molecular oxygen.
We are aerobic organisms, because we cannot survive without molecular oxygen.Both photosynthesis and the ability to
digest other organisms probably evolved much later, so we expect that the first micro organisms were chemoautotrophs,
organisms that obtained their carbon from carbon dioxide dissolved in the oceans and their energy from chemical reactions
involving inorganic chemicals.
Natural selection probably caused rapid diversification among the early lifeforms.
There were more errors in DNA copying back then because they had less sets of enzymes.
Therefore there was a higher mutation rate, and evolution happened very rapidly and life became diverse.
So tree of life branched out quickly. We can see evidence for this from microfossils.
Photosynthesis evolved from multiple steps.
1. Some organisms developed light absorbing pigments that absorbed light energy, especially UV light.
2. Overtime these pigments evolved to absorb solar energy. They used hydrogen sulfide rather than water.
3. Photosynthesis using water, producing oxygen came later and caused the buildup of oxygen in Earth's atmosphere.
Happened 2.5 billion years ago.
Having more oxygen was a problem: Oxygen attacks the bonds of organic molecules and so many species must have went
There was a metabolic process that evolved in response to this and that is how plants and animals lived until now.
Evolution of Eukarya = crucial to our evolution because we're in this domain.
More variations can happen with complex structures, this allowed for a larger selection and more advanced organisms. Eukarya came from 2.1 billion years ago,
Modern complex eukarya evolved through 2 ways.
1. some early species of eukarya may have developed special membranes that compartmentalized certain cell functions,
ultimately leading to the creation of a cell nucleus
2. some large ancestral host cells absorbed small bacterias, creating a symbiotic relationship, so both the invading
organisms and the host organisms benefited from living together.
The evidence for this relationship: two structures in eukarya that appear to be “cells within cells”, the mitochondria, the
cellular organs in which oxygen helps produce energy and chloroplasts, structures in plant cells that produce energy by
They look like tiny bacterial cells and both have their own DNA to reproduce within their eukaryotic homes.
Life remained microscopic until much later. Earliest fossil evidence for complex multicellular organisms came from 1.2
billion years ago.
The fossil record suggests that animal evolution progressed slowly at first, with relatively little change seen between fossils
from 1.2 billion years ago and those from a halfbillion years later. But then something quite dramatic happened.
Animals are grouped by body plans. This is an idea called phyla. After that we have different animal kingdoms. There is
like 30 different phyla.
Nearly all the plans suddenly appeared, it just suddenly flowered 542 million years ago and lasted 40 million years. It's
called a cambrian explosion.
Why did the Cambrian explosion occur so suddenly yet so long after the origin of eukaryotes, and why hasn’t any similar
diversification happened since? There's 4 possible answers.
1. Oxygen level reached a critical level for the survival of larger and energy intensive life forms.
2. The evolution of genetic complexity, so the eukaryotes evolved and developed genetic variation, and at this point
organisms became sufficiently complex that a great diversity of forms can evolve over a short time.
3.Climate change happened and extreme climates pressured and aided the diversification of life.
4. Absence of efficient predators, so early predatory animals were not very mature and so the stuff were given a chance to
So once predators were efficient and widespread, it became difficult for new body forms to find available environments.
Microbe life tried to find anywhere to live, and all it needed was liquid water and protection from ultraviolet radiation.
Theres plenty of areas like this on land including underground water, etc. That's how life came on land.
Some organisms had a hard time with this, since they needed to obtain water and minerals now, as opposed to just
absorbing it from surroundings.
Plants were the first on land, 475 million years ago. They evolved from alga that came from salty shallow water ponds or
The lake will dry up so these algae adapted with thicker cell walls and became used to the dryness of land and eventually
became plants and were happy because there are no animals to eat them.
Therefore, plants bloomed and developed complex bodies etc.
Animals soon followed.
Amphibians and insects began to eat land plants 75 million years ago.
Carboniferous period started 360 million years ago, and there was a lot of plants and stuff.
The land was flooded by shallow seas and dead plants decayed. The decays became layers that pressed together into coal.
Why was the rise of oxygen so important to the evolution?
We are oxygen breathing animals.
Oxygen is important because it can react with organic molecules. They offer much more efficient cellular energy
production than anaerobic processes.
Molecular oxygen = highly reactive gas.
Oxidation reactions = chemical reactions that remove oxygen from the atmosphere.
Mostly occur in living organisms that use oxygen (breathing and shit.) Before this happened mostly from volcanic gases that react with oxygen which makes rust pretty much.
It happens under the ocean a lot with the iron that there is a "red bed" that formed. This is also
why clay and rocks are sometimes red.
So where did oxygen come from? It's from microscopic bacteria called "blue green algae" aka cyanobacteria.
Fossils show they were producing oxygen 2.7 billion years ago. Maybe even before that. Our present oxygen level took 2
billion years to build.
We can learn about oxygen content from fossils, which indicates that oxygen breathing organisms existed, which means
there must have been an oxygen atmosphere, since they needed to breathe and shit.
Banded iron formations are some rock stuff from 2 and 3 billion years ago. They show that the atmosphere contained
less than 1% of the oxygen it contains today.
These are some rock stuff that formed from iron minerals that dissolved in the ocean. They only dissolve if there are
We learned that the presents of atmospheric oxygen are about 2.35 billion years old.
So basically before 2.35 billion years ago there were really little oxygen, but at 2.35 something happened and oxygen level
built up and it is called the great oxidation event.
It took so long (from 2.7 to 2.35 years ago) because non biological processes were removing oxygen from the atmosphere
as rapidly as the cyanobacteria could make it.
Once the rock and ocean minerals were saturated with oxygen, that's when the atmospheric began to build up.
What happened wen the event happened? Apparently there was a slow buildup and we don't know why.
Anyways, it seems like oxygen level was too low for complex animals until the Cambrian explosion, and then oxygen
breathing animals evolved at that time.
200 million years ago there was finally enough oxygen for fire to burn.
If you had a time machine and could randomly spin the dial to take you back to any point in Earth’s history, you’d have 1 in
10 or 1 in 20 chance of appearing at a time recent enough to have air.
The fact that it took so long for oxygen to build up in the atmosphere on Earth should make us wonder about the likelihood
of getting oxygenbreathing life on other worlds.
Maybe we just got lucky. Or unlucky that it took so long. (If nothing was preventing oxygen build up in other planets
maybe there will be animals around 1 to 2 billion years instead of 4 billion)
6.4 Impacts and Extinctions
245 million years ago there were dinosaurs and mammals.
There were some transitions that were not every smooth.
One was the transition between Cretaceous and Tertiary periods, 65 million years ago, when dinosaurs disappeared.
There has been more than 150 impact craters on our planets.
KT boundary is this CretaceousTertiary boundary found in Italy.
Basically they found rich iridium elements.
That is when the Alvarez team proposed that the extinction of the dinosaurs was because of the impact of an asteroid or
It was an asteroid 1015 km in diameter, 65 million years ago.
99% of all living plants and animals died, and 75% of all existing plants and animal species were driven to extinction.
This is called the mass extinction.
Evidence for this comes from the KT sediment layer.
Evidence are: high level of other metals, grains of quarts crystals that indicate high temperature pressures of an impact,
spherical droplets of molten rocks that cooled and solidified, and soot that appeared from forest fires.
These features are different from the rest of Earth's surface, except other impact sites like the Meteor Crater in Arizona. On that fateful day some 65 million years ago, the asteroid or comet slammed into Mexico with the force of a hundred
million hydrogen bombs.
It apparently hit at an angle, sending a shower of redhot debris across the continent of North America. A huge tsunami
sloshed more than 1000 km inland.
Much of North American life may have been wiped out almost immediately. Not long after, the hot debris raining around
the rest of the world ignited fires that killed many other living organ isms. Indeed, the entire sky may have been bright
enough to roast most life on land.
Dust and smoke remained in the atmosphere for weeks or months, blocking sunlight and causing temperatures to fall as if
Earth were experiencing a global and extremely harsh winter. The reduced sunlight would have stopped photosynthesis for
up to a year, killing large numbers of species throughout the food chain. This period of cold may have been followed by a
period of unusual warmth: Some evidence suggests that the impact site was rich in carbonate rocks, so the impact may
have released large amounts of carbon dioxide into the atmosphere. The added carbon dioxide would have strengthened the
greenhouse effect, so that the months of global winter immediately after the impact might have been followed by decades
or longer of global summer.
The impact probably also caused chemical reactions in the atmosphere that produced large quantities of harmful
compounds, such as nitrous oxides. These compounds dissolved in the oceans, where they probably were responsible for
killing vast numbers of marine organisms. Acid rain may have been another byproduct, killing vegetation and acidifying
lakes around the world.
Some small mammals lived. This is b/c they lived underground and stored food to last through the global winter.
Now mammals became kings of the planet, and rapidly evolved into larger mammals and eventually humans.
Did impacts cause other mass extinctions
There was 5 major extinctions, including the KT extinctions.
On average, impacts the size of the K–T event should happen about every 100 million years or so, which is roughly the
same as the average time between mass extinctions
Other than impact craters, some ppl think that maybe these extinctions were caused by active volcanism or a variety of
factor that is called the "sick earth hypothesis"
Or maybe it was because of mutation rate changes. Because ozone layer becomes thin sometimes, UV light caused a lot of
mutation like cancer.
Or mutation is called by high energy particles that streams from the Sum. They usually don't reach earth because of
Earth's magnetosphere. Sometimes it's weak or reversed or disappear and spike up mutation rate.
Some scientists hypothesize that more distant events could trigger mass extinctions on Earth, including supernovae, the
explosions of massive stars.
This creates cosmic rays and then when it occurs near Earth, we might expect a spike in the number of cosmic rays
reaching Earth, which will cause mutation.
Also mass extinction might be due to gamma ray bursts, produced by powerful supernovae.
It's powerful enough to destroy half of Earth's ozone layer, which will lead stuff to die because of exposure to UV light.
Human activity is driving species to extinction so rapidly that half of today’s species could be gone within a few centuries
Is there a continuing impact threat?
Small particles hit Earth almost continuously, and they burn out at the atmosphere and we call them meteors or shooting
Size of a pea and goes at high speed, burning surrounding air to produce the flash.
Some bigger ones heat to the point of explosion and the flash is called a fireball. The dusty shit that ends up falling onto
Earth we call them meteorites.
Sometimes they hit earth but are unnoticed because they land in remote areas. This only happens every century or so. If an asteroid or comet comes towards Earth we are not sure if our technology is up for the task of demolishing or diverting
6.5 Human Evolution
We are primates and our ancestors lived in the trees,
We swung around trees and our eyes are close together to provide overlapping views.
Parent child bonds exist and thats why our babies are so useless as opposed to other species.
We did not evolve from gorillas, we just share a common ancestor.
Hominids is the term for ancestors of humans, and we followed a complex path.
There might have been many species and many just reached dead ends.
Toumai is a fossil that dates to 6 and 7 million years ago of a lineage of this thing thats like a combination of apes and
Ardi is this thing that lived 4.4 million years ago and it shows upright walking, so it's probably our ancestor.
The earliest fossil skulls that look essentially like those of modern humans are about 100,000 years old.
Even the, our ancestors shared the planet with other 2 hominoid species.
The Neandertals were quite similar in appearance and brain size to Homo sapiens, and excavations of sites where they
lived indicate they had culture, arts, and possibly religion and speech.
The Neandertals disappeared for unknown reasons about 30,000 years ago, but their genes may still survive:
Up to 4% of the modern human genome originated with the Neandertals, which means that Homo sapiens and Neandertals
must have interbred.
Homo floresiensis lived around Indonesian 12 000 years ago. They were a meter tall and named the hobbits.
People with different skin colour and appearance, but our genes are nearly identical.
All this shows that relatively small genetic differences can make a big difference in species success. More than 98% of the
DNA sequences that make up the human genome are identical to the sequences that make up the chimpanzee genome.
This also suggests that the evolution of intelligence is a complex process. Is advanced intelligence an inevitable outcome of
Are we still evolving?
The changes during the past 10,000 to 40,000 years have probably been relatively small.
Nevertheless, we have clearly gone through dramatic changes as a species.
These dramatic changes are not due to biological evolution, but rather to what we might call cultural evolution—changes
that arise from the transmission of knowledge accumulated over generations.
Compared to chimps and apes, humans are special because reached the point where cultural evolution is far more
important to our changing nature than is biological evolution.
Biological evolution = driven by random mutations, therefore it takes a steadier, slower process.
Cultural revolution accelerated, and recently we accelerated into a technological evolution.
Technology allows us to reengineer living organisms and we will soon outpace nature in developing species.
So if we can manipulate DNA. we can "improve" our specie. However, there is moral and ethical restrictions.
6.6 Artificial Life
Here we'll talk about lab experiments that seek to understand origins of life.
Some go further, trying to recreate life.
This is an attempt to put together novel organisms that can reproduce and grow, but on a microbial scale.
If we succeed, it will mean that creation of life is a straightforward chemical thing.
Scientists try to create artificial life by rearranging bits and pieces of existing organisms or trying to build an extremely
simple living cell in the lab.
Craig Venter: I tried to make a top down program to to make designer organisms. I wanted to create things that were
useful and we use existing bacterias and trip out the genes. Then I build up from this minimalist gene using short DNA
sequences. I insert them into the cell and the cell came to life and reproduced. It's kinda useless for now but the concept is
there. Jack Szostak: I studied genes and tried to make RNA based cells that replicate. I picked RNA strands and gave them the
chance to react chemically. They did indeed reproduce. I trapped RNA strands inside bubble like membranes of fatty acid
so it's almost a cell like environment.
Should we create artificial life?
Although we can make good organisms that cures cancer, etc. but someone can use it to make deadly organisms that's
The ethical dilemma is huge, if humans have the moral right to create new types of life?
They solved this by using computer software as opposed to the real thing. These software gives us a chance to determine
the effects of new drugs and the consequences of adding or removing genes, etc.
The future of biological science— and perhaps of our species—will depend largely on the ethical choices that we make in
this technological evolution.
CHAPTER 7: SEARCHING FOR LIFE IN OUR SOLAR SYSTEM
Because there are many places to look, we need a strategy to help focus our efforts on the worlds most likely to be
First we need to know the environmental requirements that we expect to be necessary for life on any world. Then we seek
to find places that meet these requirements. And eventually we decided where we should focus on.
We are looking mainly for microbes or other simple life.
Even if we can't find any in our own solar system, the search itself will teach us about characteristics that can make a
planet habitable and will thereby help us when we extend the search to other planetary systems.
7.1 Environmental Requirements for Life
We can't survive in other worlds because they have no atmosphere to protect us from ultraviolet radiation and surface
But looking for microbes and other life broadens the possibility of life in other worlds.
Where can we expect to find building blocks of life?
Perhaps the most obvious requirement for life is a set of chemical elements with which to make the components of cells.
Life on Earth uses about 25 of the 92 naturally occurring chemical elements.
Four of these elements—oxygen, carbon, hydrogen, and nitrogen—make up about 96% of the mass of living organisms.
This requirement can be easily met.
All elements were produced by stars, except hydrogen and helium. These elements are called heavy elements and are
harder to find, but still they're everywhere.
Every star system we've studied has at least some amount of all the elements used by life.
According to the nebular theory of solar system formation, the planets were built when solid particles condensed from gas
in the solar nebula, and these particles then accreted into planetesimals and ultimately into planets, moons, asteroids, and
The first step in this process—condensation— affects only the heavier elements or hydrogen compounds containing heavy
elements, because pure hydrogen and helium always remain gaseous. As long as condensation and accretion can occur, we
expect the worlds will contain elements needed for life.
Life on Earth is carbon based and so we think life elsewhere would also be carbon based.
Although there's the chance that it's not the case, but we think we're right since the elements that produces stars are the
same in everywhere.
No matter what kind of life we are looking for, we are likely to find the necessary elements on almost every planet, moon,
asteroid, and comet in the universe.
However, theres stricter requirements for the presence of these elements in molecules that can be used as building blocks
Recall that Earth’s organic molecules likely came from some combination of three sources: chemical reactions in the
atmosphere, chemical reactions near deepsea vents in the oceans, and molecules brought to Earth from space. The first two sources can occur only on worlds with atmospheres or oceans, respectively. But the third source should have
brought similar molecules to nearly all worlds in our solar system.
Studies of meteorites and comets suggest that organic molecules are widespread among both asteroids and comets.
However, organic molecules tend to be destroyed by solar radiation on surfaces unprotected by atmospheres.
Moreover, while these molecules might stay intact beneath the surface—as they evidently do on asteroids and comets—
they probably cannot react with each other unless some kind of liquid or gas is available to move them about.
Thus, given that it makes sense to start our search with worlds on which organic molecules are likely to be involved in
chemical reactions, we should concentrate on worlds that have either an atmosphere or a surface or subsurface liquid
medium, such as water, or both.
Where can we expect to find energy for life?
In addition to a source of molecular building blocks, life requires an energy source to fuel metabolism.
Life on Earth uses a wide variety of energy sources. Some organisms get energy directly from sunlight through
photosynthesis. Others get energy by consuming organic molecules.
The energy available in sunlight decreases with the square of the distance from the Sun.
At 10 times Earth’s distance from the Sun—roughly the distance of Saturn—it would receive only 1/(10^2) aka 1/100 of
the energy it would receive on Earth.
Photosynthetic life on such a world would have to be either much larger than life on Earth (giving it a larger surface area
for collecting light), much more efficient at collecting solar energy, or much slower in its metabolism and reproduction.
In the far outer solar system, sunlight almost certainly is too weak to support life.
Chemical energy sources also place constraints on life. Chemical reactions can occur under a wide variety of
circumstances, but only if the potential reactants are brought into contact with each other.
This means that the ongoing reactions needed to provide energy for life can occur only on worlds where materials are
being continually mixed.
On a practical level, this probably requires either an atmosphere to mix gases or a liquid medium to mix materials on or
below a world’s surface—the same requirements we found for obtaining the building blocks of life.
Does life need liquid