探索过去的45亿年 更好地理解空气，水，土壤和生命如何形成 及相互影响
Video 4.3.1 The Formation of Organic Molecules and the Tree of Life
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來自 University of Manchester 的課程
我们的地球：气候，历史和进程
70 個評分
從本節課中
Life, and its Effect on Earth’s Climate System
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Prof. David M. SchultzProfessor of Synoptic Meteorology
School of Earth, Atmospheric and Environmental Sciences
Dr Rochelle TaylorPostdoctoral Research Associate
School of Earth, Atmospheric & Environmental Sciences
Dr Jonathan FairmanPostdoctoral Research Associate
School of Earth, Atmospheric and Environmental Sciences
Hello, welcome to Our Earth, Its Climate, History, and Processes.
My name is David Schultz.
In this lecture, we're going to talk about Early Life.
And this is going to be from the period around 3.8 to 2.1 billion years ago.
As a general framework for the timeline that we're talking about,
let's remind ourselves that the earth formed four and a half billion years ago.
And by 3.8 billion years ago,
we had the first water-lain sediments produced by, the ocean.
By 3.5 billion years ago,
we have the first fossil evidence of single-cellular life.
It took almost 3 billion years until multicellular life formed.
The earliest fossils we have of those are 630 million years ago.
So let's think about that, shall we?
So for 3 billion years, out of the 4 and a half billion years that the Earth
has been around, the only form of life on the planet was single-cellular life.
The evolution to multi-cellular life took nearly 3
billion years after single cells first were observed in the fossil record.
So why is this?
Why did it take so long, and how sensitive was this, evolution,
to the environment that perhaps drove the formation of multicellular life.
This is what we will explore in the next lecture.
So, how do we get from a situation where there is no life on Earth,
no organic substances, to single cellular life?
One of the most interesting and exciting experiments in the history of
science was the pre-biotic soup experiment by Miller and Urey in the 1950s.
They conducted experiments to investigate how inorganic materials could
produce the substance of life.
They designed this closed chamber where they placed a mixture of
the gases that they believed at that time the primitive atmosphere was composed of.
So, methane, ammonia,
the water vapor from the boiling water below and molecular hydrogen.
And then, within this, volume that represented the model atmosphere they
had two wires and then they would send an electric spark across these wires.
This spark represented the lightning that may have been present in the early
Earth's atmosphere.
And then after some days of running this experiment, they looked at
the substances in the water and found that they were able to produce amino acids,
the building blocks of proteins, out of inorganic substances.
So this was a big revelation that you could get from non-biotic substances and
create organic materials that would serve as the building blocks of life.
Now, when they did this experiment, they chose substances that
they believed the primitive Earth's atmosphere would have contained.
But science has progressed and
we've learned more about what this early atmosphere may have looked like.
And so, now you can do this experiment and
put more realistic primitive atmospheres inside this glass jar.
So you might include carbon dioxide, or, molecular nitrogen or
hydrogen sulfide and sulphur dioxide.
Stuff that would have been emitted from,
the out gassing of the early Earth at this time.
And when you do this you get even more complex and
diverse organic molecules that are formed inside this soup.
So the implication is clear, that if you have components inorganic components that
resemble what the Earth's early atmosphere may have looked like, and then create
an electric spark to initiate chemical reactions, then you can form amino acids.
Later experiments showed that, as soon as you put oxygen into this environment,
then you lost the ability to create the amino acids.
The oxygen prevented the amino acids from forming in this environment.
So we know that, in an anoxic atmosphere, which is the way we believe that the early
Earth's atmosphere existed in, then these amino acids could have been produced.
So, that's how we can get organic substances out of inorganic components.
Eventually what would happen is that you would
create what's called the universal ancestor.
This is the first organism from which all life evolved.
So it had to have two characteristics.
First of all, it had to be composed of carbon rich compounds.
That's the definition of organic life on our planet.
Then the second thing is that it would have had to have
had genetic information for growth and reproduction.
And so this genetic information was likely RNA, maybe DNA that
would've acted to cause the molecules to reproduce.
And then thus create the first micro-organism.
Now this universal ancestor is significant because we recognize that the nucleic
acids and, the building blocks, the amino acids, are characteristic of life.
Life is built up from these amino acids.
And so everything that lives on Earth has this basic architecture.
And that leads us to believe that there is something at least
worth discussing about this universal ancestor.
Before we discuss how the universal ancestor gave birth to all of
life on Earth, we need to introduce a few definitions.
The first definition is called a prokaryote.
A prokaryote is mostly a single-celled organism that lacks a nucleus and
other internal organelles.
On the other hand,
eukaryotes are organisms that have cells that contain the nucleus with DNA.
And then, finally, there are metazoans.
These are, mostly eukaryotic multi-cellular organisms.
And so those are organisms that are composed of more than one cell.
If we do a comparison between prokaryotes and eukaryotes,
we see that prokaryotes the DNA is spread throughout the, internal cell.
But within a eukaryotic cell, the DNA is housed within the nucleus and
there are other organelles that are imbedded within the cytoplasm around the,
around the, the nucleus and within the cell wall itself.
So when we look at all of life on Earth and
scientists can classify this into what we call the tree of life,
then we end up with three different domains.
These are differentiated by the types of cells that exist.
Both bacteria and archaea are prokaryotic cells.
Despite this similarity, archaea are more closely related to the eukaryotes,
because that is the way that these chromosomes undergo translation and
transcription, which is different than how that works in bacteria.
So, that's why we see that archea tends to be more closely related
to the eukaryotes, even though they're both prokaryotic cells.
We also know that organisms that make methane are only found in archaea.
Now, based on these definitions, scientists have classified all of living
life into one of three domains, either the bacteria, the archaea, or the eukaryotes.
And these three domains then represent all of life.
Both bacteria and archaea are prokaryotes and eukaryota is the eukaryotes.
And despite this similarity between the bacteria and the archaea,
we know that archaea are more closely related to the eukaryotes because that's
the way that their chromosomes undergo translation and transcription,
which is different in the way it happens in bacteria.
So that's why archaea is more closely related to the eukaryotes.
When we look at the ways that organisms, within each of these three
domains metabolize, we see that within the bacteria,
bacteria can metabolize through photosynthesis, through aerobic and
anaerobic respiration, fermentation, and autotrophy.
Eukaryote, on the other hand,
only metabolize through photosynthesis and cellular respiration.
So that would be the plants and animals.
The archaea tend to produce their energy from inorganic substances.
So, for instance, organisms that make methane are only found in the archaea,
so they take the elements from the environment then produce methane.
So, given this definition then,
we want to look at the types of organisms that would've been around
shortly after the universal ancestor and led to this expansion of the tree of life.
Given the fact that the environment was going to be quite
hostile in the early atmosphere.
There would have been no ozone layer, so
the Earth would have been pounded by intense radiation.
The temperatures in the oceans may have been quite warm.
And certainly they were without oxygen.
So extremophiles are classified as organisms that live on the edge.
These are environments that would kill other organisms.
Whether it is by, being too salty, the halophiles.
Being too acidic, the acidophiles.
Being in the absence of oxygen, the anaerobes.
Or, excessively hot temperatures, in other words, the thermophiles.
So these extremophiles were likely some of the earliest forms of life.
And we can see some of those around today.
We have here in the Grand Prismatic Spring in Yellowstone,.
Thermophilic bacteria, bacteria living at very high temperatures,
temperatures ranging anywhere from 30 degrees up to 70 degrees Celsius.
And the different colors of the Grand Prismatic Spring
represent different ranges in which different organisms are living.
Near the center of the hot springs the water is going to be
quite hot and the blue color there is simply due to the fact that
that's the natural color of the water and there is not much living in there.
As you get further towards the outer rim of the spring and
the water becomes progressively cooler.
We see different colors being displayed by these single celled organisms.
And as you get even out further,
the brown bacteria that live along the edge fix nitrogen from the air and
live in water temperatures that are not much different than 30 degrees C.
And their dark brown pigment protects them from
the ultraviolet radiation that we get at this high altitude.
The organisms that lie towards the center are either photosynthetic or chemotrophic.
