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AP®︎/College Biology
Course: AP®︎/College Biology > Unit 7
Lesson 11: Origins of life on earthOrigins of life
An overview of the question of how life first arose on Earth. Created by Sal Khan.
Want to join the conversation?
- At, what does he mean with 'non-living organisms'? If something is not living its not an organism is it? 3:02(5 votes)
- A virus is an example of a "non-living organism." Viruses are sometimes called non-living because they can't replicate their own DNA. They have to invade a cell and use the cell's machinery in order to replicate themselves. As Sal said, viruses are much less complex than "living" organisms.(9 votes)
- As Sal says atand 8:48that RNA is able to replicate itself and catalyze actions, How is it possible, I mean how does it do that? 9:00(3 votes)
- This is possible, and it is the idea behind the RNA world hypothesis (which states that self-replicating RNA gave rise to life on earth). Scientists have synthesized small molecules of RNA called ribozymes (RNA enzymes) that act as enzymes to replicate other strands of RNA. They consist of the same chemical compounds as other RNA's, but tend to have more complex tertiary (non-linear) structures.(5 votes)
- What is the difference between replication and metabolism?(3 votes)
- DNA replication is the first step of the Central Dogma of Molecular Biology. The process by which DNA is duplicated.
Metabolism is a complex process of a series of chemical reactions that are interdependent and interconnected in our bodies.(2 votes)
- I really liked this video. Never heard of the "metabolism first" theory before. It made me remember back to a much earlier video where Sal describes entropy not as the "measure of chaos" but rather as the number of possible states. Chaos suggest disorder, whereas number of states suggests potential organization.
My question is: does life at deep sea vents have any common ancestor with surface life? Do photosynthetic organisms or the eaters of those carbon fixers, have any common ancestors with chemoautotrophs? IE: Did life evolve more than once in our planetary history?(3 votes)- Firstly, It would be incorrect to use the term evolution to refer to the emergence of life. Secondly, to answer your question, I believe that the scientific consensus is that all life on earth is almost certain to have evolved from the same common ancestor.(0 votes)
- At- 0:34, Sal Khan states that we know the earth was formed about 4.7 BYA. How was this number found? 1:03(2 votes)
- Is it possible for more life than us to be around?(1 vote)
- how did life emerge in this planet but not in any other planets in our solar system ??(1 vote)
- Earth just got lucky, I guess. We had water that was not frozen. It is believed that most of our water came from asteroids hitting the newly formed earth. This water vaporised and formed clouds, insulating Earth and allowing Earth to cool down, and then the clouds themselves condensed. So the water didn't freeze and condensed and stopped being water vapor. So Earth, by chance, happened to get an atmosphere that allowed for liquid water, and the neat thing is that there were also other molecules coming out of hydrothermal vents under the oceans, which meant a fresh supply of nutrients that allowed the life-sustaining molecules to come together and create the very first cell.(1 vote)
- What did the Miller-Urey experiments initially yield?(1 vote)
- Urey-Miller experiment in 1953 exposed hydrogen, steam, methane and ammonia to an electrical discharge. The initial results included far fewer organic molecules – only five amino acids.(1 vote)
- How hot would it have been during the Hadean Eon? Do you think that if Water Bears were there, would they have been able to survive?(1 vote)
- They of course would if they could swim and find prey in molten lava. Jacob, have you ever been to a glass manufacturing industry. If you do visit one try and put your hand in the molten mixture. It is that ridiculous for seals, or water bears, to survive there. I doubt if there was any existence of liquid water back then.(2 votes)
Video transcript
- [Voiceover] We have many
videos on Khan Academy on things like evolution
and natural selection. We think we have a fairly
solid understanding of how life can evolve
to give us the variety, the diversity, and the complexity that we've seen around us, but it still leaves unanswered
a very fundamental question. And this might be the
biggest question known to us, and that is the origins of life. How did life first
emerge, at least on Earth, and that even starts to
lead to other questions about is there life outside of this planet and what could it be like? And so let's start with
what we actually know and I'm gonna start with a timeline. So let's go one billion years ago, let's go two billion years ago, three billion years ago, four billion years ago. So this is now. And once again, we're talking
about a billion years ago. You'll sometimes see the abbreviation BYA, billion years ago, which
is an unfathomable amount of time going into the past. But we know that Earth along with the rest of the solar system was formed around 4.6, 4.6 billion years ago, so that's when Earth was formed. And right at 4.6 or
even, you wait a casual 100 million years after
that 4.5 billion years ago, we believe that Earth wasn't very suitable for even very simple life to form, and that's because the solar
system was a crazy place. You had collisions of all scales
happening all of the time. The moon itself was
formed from the collision of two planet-sized objects,
one we call it the proto-Earth and another planet-sized
object and they collided and then they started to spin around and one part became the moon. It was tidally linked with the Earth. But you can imagine,
that's not an environment where it would be easy for life to form. And even once the moon was formed, you had a heavy bombardment of things in the solar system, the solar
system was a messy place. It took a long time for the stability that we now observe out there. And so that continued, we believe, until about 3.9 billion years ago, which is the earliest
that we currently think that Earth might have
been suitable for life. Before that, there might have been pockets where the bombardment
stops and maybe some type of primitive life might have formed, but then they would have gone away with the heavy bombardment. But who knows? Maybe they could have
survived that somehow. But that's the current mainstream belief. The other thing we know is
that we see fossil evidence for life 3.5 billions years ago. And these are stromatolites. This is fossil evidence, microorganisms, they formed these structures
that actually continue to be formed today,
these types of structures continue to be formed today. And although it might not
feel like microorganisms are complex life, when you
think about what has to happen within a microorganism, they're
actually incredibly complex, and especially if you compare them to very simple, non-living organisms. So our current belief is someplace in this region life must
have arisen on Earth. But that still doesn't,
even if we were able to answer that question, oh,
it was exactly 3.7 billion years ago was the first
time that some RNA decided, or not decided, ended up getting
in the right confirmation so it could replicate itself in some way, even if we know that date,
it still leaves unanswered maybe the more interesting
question, which is the how. The how is really, at least
to me, more important, more interesting than the when. And to the how question, there's
a couple of layers on it. The first is, let's just
start with the most simple molecules that we would
have expected to find on early Earth. Here are some examples
of it right over here. This is H2O, or more
commonly-known as water. Right over here is CO2, more
commonly-known as carbon. That's a little hard to see, let me do it with a lighter color. So we have carbon dioxide right over here. Here we have molecular
nitrogen, you have some ammonia, you have some phosphate, and
many other of the elements that we see on Earth today,
they might have been available in that early Earth, but
how do they form at least, even the next step up, which
is the slightly more complex, or actually a good bit more
complex organic molecules. And when people talk about
organic molecules they might be talking about things like this. These are amino acids. These are the building blocks
of proteins, amino acids. You see over here nucleotides. These are the building blocks
of RNA, DNA, other things. And so the first question is, and these aren't the only
simple organic molecules. You could think about sugars
and all sorts of other things. But the question is, is it realistic? Do we at least understand how we can go from these very simple molecules up here to these more complex, often
called organic, molecules? And the simple answer is we
now have a lot of evidence that this is doable, that you can go from these
things to these things, abiotically, without the presence of life. You'll hear that word abiotic a lot. Think about it, antibiotic,
you're killing life, you're killing bacteria. Abiotic, that is without life. And the points of
evidence that we now have are we believe and we've seen evidence that there's amino acids and
organic molecules related to them on comets,
meteorites, on other planets, that they formed spontaneously in space, once again, without the
presence of life there. We've even been able to form amino acids and other molecules like this from these more simple
elements in the laboratory. The most famous experiment there is the Miller and Urey Experiment. This was in the 1950's,
where they were able to show with some energy, they provided a spark. You could imagine that in the early Earth it could have been from lighting. And they tried to set up a mix
of gases that they believed was similar to the atmospheric
mix in the early Earth which didn't have much oxygen
in the atmosphere then. We needed life to
actually start to produce some of that oxygen. And even though today we think that they probably didn't
have the mix of gases right, they did do something significant. They were able to show
that with that mix of gases at least they thought
were in that atmosphere, and some energy being
added to that system, that they were able to form some of these organic molecules. So we should feel pretty good that at least this first step is doable. Now the next question is
these organic molecules by themselves, that's not life. In fact, these aren't even
the most complex molecules that are, we believe, essential for life. Proteins are where things start
to get really interesting, and a protein, a protein, or proteins are one of the places, a protein might have
thousands of amino acids, thousands of amino acids. Things like DNA and RNA,
also we believe essential for life, or at least life as we know it, could be made up of tens
of millions of nucleotides for one DNA molecule. So for example, this is just a small part of a DNA molecule, but
you can already see much, much more complex than
what we see over here. And there, too, we have
evidence that you can go from the amino acids to the proteins, or you can go from the
nucleotides to the DNA without the presence of life, that these things can happen spontaneously if you have the right context,
the right energy available, some people believe,
or it's been observed, that if you have the right surfaces that these molecules can be
organized in the right way to form these more complex things. Now, I know what you're thinking. Alright, proteins are really cool, DNA, RNA is really cool, but
then how does that become life? At what point would we start going, "That was a proto-life form?" And this is where we
really get into the area of the unknown because we don't know. And there's a couple of
hypotheses out there. One of them is called
the RNA World Hypothesis. I'll write that down. RNA World Hypothesis. And this is the idea
that the first proto-life was self-replicating RNA molecules. And the reason why people tend to focus in on RNA a little bit more than DNA is that even in cells today, RNA doesn't just store information, it can actually play a role as a catalyst. And when you think about things like tRNA and you think about ribosomal RNA. And so maybe some of that
first proto-life was RNA, information that replicated itself and catalyzed the replication of itself. Maybe it somehow got organized into membrane-bound structures
so it could separate so you had environments
that were separated from the outside world. But the simple answer is we don't know. Another mainstream hypothesis is the Metabolism First Hypothesis. Metabolism Metabolism First. And this is the idea that
a lot of basic pathways that you might study
in a biochemistry book, that these were first
just happening, well, all of this could have been happening in this primordial soup where you had these organic molecules
in the right conditions, maybe around heat vents and whatever else, but the Metabolism First is
that some of these mechanisms that we now study in biochemistry, these might have happened
outside of a cell or outside of life and they
just kept creating more and more complexity, but
at some point these things started happening in self-organizing, membrane-bound structures. Maybe there's some kind
of combination of the two. The simple answer is we just don't know, but there's some fascinating clues. Even if we observe current biology, and even in fact if we
see the commonalities of things that happen,
central dogma of biology, if we see how proteins,
which structures are common to all life as we know
it, it might give us clues or hints at what some of
that very earliest life or proto-life was actually like.