Practice English Speaking&Listening with: Origin of Life

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Hello

and welcome to this course calledBiology for Engineers and other Non-Biologists”.

My name is Madhulika Dixit and I am a faculty at Department of Biotechnology, and I am taking

this course along with my colleague, Professor G K Suraish kumar. Now when this idea of taking

this course for engineers and non-biologists was brought up to me, the idea was to essentially

bring in biology, teaching biology at a level which will be easily taken up by the non-biologists.

So for those students who have already taken some sort of a course in biology, a lot of

things will sound very fundamental. However, the whole idea of this course is to essentially

highlight the basic features of biological life.

Now, most of the times when I have taken these kind of courses with engineering students,

I have always noticed an apprehension in terms of trying to learn a biological course, and

the reasons that I have gotten more often from them is that it's a subject which requires

a lot of memorizing. I would like to bring out to the students here that indeed there

are lot of terms which seem very difficult, but I would also like you to understand that

biology is at a very infant stage, in comparison to, for example physics or chemistry, where

the logics of chemistry and physics are very well defined. For biology, we are still trying

to understand those logics.

Hence, in this course, well try to understand the logics of how a biological life is governed,

and what is it in terms of reactions, in terms of physical entities, which help a life to

survive and sustain. So let me start this course with my first lecture or rather first

set of slides on origin of life. Now, life, or origin of life is a very intriguing topic

because for a very long time, we still haven't been able to answer the question as to how

life really evolved. But before I get into the actual topic of today, which is origin

of life, it's important to understand what are the features of life. What is it that

you calllifeand how is it sets itself apart from non-living things.

Well, anything that you see around yourself which is living, you will notice that there

are certain features which are fundamentally common across living world, whether you start

from a bacteria, you go to a plant, you go to a fungal organism or you go all the way

to human beings,

you find that there are certain features of life which remain constant, and the most important

of them all is the ability to replicate and reproduce. Now this is one critical feature

which sets the living world separate from the non-living world. But in order to do that,

in order to sustain it's survival, in order to pass on it's characters to it's progeny

or it's off-springs as we call it, an organism goes through a whole lot of processes, and

these include it's ability to utilize food, it's ability to synthesize food if the need

be, it's ability to digest the food, it's ability to throw out the waste material and

keep the machine going.

So in other words, life or any biological system is a very highly dynamic system, and

it has it's sole purpose of surviving and reproducing. So, this is one character of

life which is very unique, and we are still puzzled and we still dont have a complete

answer as to life, how a life really evolved on this earth. But, lot of studies done by

geologists, by archaeologists, by molecular biologists, have noticed that there are enough

proofs as of today, with which we can confidently say or at least speculate that the present

form of life has evolved from non-living things. In other words, we have essentially evolved

from chemical reactions.

So, lets get to how the history of earth is, and how life really evolved during this

history of earth. Now if you notice in the slide, if you were to look at the life of

earth as a whole, and here Ive drawn a clock and the time scale or in terms of billions

of years, what youll notice is that the radio active dating estimates that the origin

of earth happened somewhere close to four point

eight billion years ago, and, the earliest fossil records which suggest existence of

life forms is about a billion years later. So, it is possible for us to speculate and

now you have enough proofs through fossil records that the life must have originated

within seven hundred to eight million years of earths existence.

So, how did the life evolve is something that well talk in this class today, but I want

you to understand that a lot of this was initially a set of chemical reactions, and these set

of chemical reactions were actually possible because the earths atmosphere was highly

conducive for such reactions to happen. And mind you, at that point of time in the early

life of earth, the atmosphere of the earth was very different from what we see of today,

which is essentially full of atmospheric oxygen. So in other words, the initial phase of earths

history, it had a highly reducing environment. And Ill come back to this when we talk

about experiments which actually go on to prove how the initial biological molecules

actually got synthesized.

So, the earth seems to be about 4.6 to 4.8 billion years old, the earliest fossil records

are somewhere about 3.6 to 3.8 billion years old, and somewhere in between the life really

evolved. And what we also observe is one of the earliest forms of lives are something

which we even see them today and those are the bacterial forms. So it's interesting that

something which evolved or which came into existence more than three billion years ago,

has sustained it's survival for the last 3.5 to 4 billion years.

And in the process, the Prokaryotes have also evolved and has given rise, is what we think

as of today to different forms of life. So well come back to this and see it in a

step by step fashion.

So, for simplicity, what we understand today or rather, based on the various evidences

that we have from geological excavations, from chemical reactions, from molecular biology

techniques, for simplicity's sake the biologists believe and have divided this very process

of life and it's existence or origin into three different phases. The very first phase

is of chemical evolution. Now this must be the phase which must have evolved during the

very early stages of earths life, when the earths crust was very hot, the material

has still not cooled down, the oceans, the springs, the hot pools were still boiling,

the atmosphere was highly reducing.

And, it is during this phase that probably some sort of geological complexes or molecules

would have interacted to form the initial and the early very building blocks of organic

molecules. Well come to this. Then came the most important feature which kind of sets

apart life from chemistry, and that is the ability of these molecules, somewhere during

the course of history, to develop into a property where it can replicate itself.

Now this is one of the most critical features, I would say, in terms of the origin of life,

because it is at this stage you would find, that probably

the system changed from just a set of chemical entities into developing a property where

they could self-replicate.

And then, comes the last phase which would probably correspond to the rest of the phase

all the way from here till what we see present today, is the evolution. And, what do we mean

by evolution; it was just initially a set of chemical entities, all sitting together,

these set of chemical

molecules must have evolved an ability to self-replicate, and then sooner or later,

these entities would have enclosed themselves into organism-like entities, which were the

initial prokaryotes, and then the prokaryotes would have evolved into much more complex

forms of life, which is what you call as the eukaryotes, a single-celled organism to a

multi-cellular organism to plants, to animals, and then, to the present day human beings.

So, in that sense, for simplicity, we kind of divide this development of life into three

phases, chemical evolution, acquisition of the replicative ability by life, and the evolution

of these early forms of life into what we see today as complex organisms and complex

plants.

So lets go to what are the various theories and evidences for origin of life. But before

I get into the theories and evidences of life, I want to highlight certain things, which

are very important to understand how these theories were built up. If you were to look

at any living organism as of today, we all know that our, seventy percent of our body

weight is made up of water.

But if you were to just account for the dry weight, youll find that the ninety percent

of a dry weight of any living being essentially consists of these major elementsthe most

abundant being the carbon, hydrogen, oxygen, nitrogen, phosphorous and sulphur. In addition

to these, you do have some trace elements like iron, copper, zinc; but in terms of bulk

quantity, the bulkiest, or the one which is available in most quantity is the carbon,

and that should not be a surprise looking at the chemical properties of carbon, because

carbon has a versatile ability to form

covalent bonds, not just with itself, and thus lead to a infinite long chains of organic

molecules, it can also form covalent bonds with other elements, like hydrogen and so

on.

So in that sense, carbon seems to be a very nice element. And, it should also be noted

that when geologists went on analyzing the chemical composition of the early rocks of

earth and the meteorites, which continue to keep hitting us, they were found to be very

rich in carbonaceous compounds. So clearly, the earliest building blocks were formed because

of carbon. The other important thing to note is that the initial earths atmosphere,

which is what we call as the primordial earths atmosphere, was highly reducing. As I mentioned

earlier, it is in, in the initial seven hundred to eight hundred million years, you would

find that the earths atmosphere was highly reducing because of presence of nitrogen,

ammonia, methane, carbon dioxide, and it actually did not have any atmospheric oxygen.

So, what was the first hypothesis? With this background in mind, the first hypothesis was

put forth by Alexander Oparin and J.B.S Halden in 1929, and they said, that the very first

molecules, or biological molecules would have arosen, because of the abiotic synthesis of

small molecules. So the first set of sugars or amino acids would have been formed by a

mere abiotic synthesis of these molecules. And how it would have been possible? According

to them, in that initial reducing environment of the earth, any high energy discharge, either

in the form of ultra-violet lights, or in the form of lightning would have favoured

spontaneous synthesis of simple molecules from existing geological molecules on earths

surface.

Now this was just a hypothesis way back in nineteen twenty-nine and twenty-four years

later, in 1953, it was actually demonstrated to be true by Stanley Miller and Harold Urey.

So what did Stanley Miller and Urey did is that they tried to create this kind of a primordial

earth atmosphere, or the primordial soup, in this case I mean the early forms of oceans

on earth which must have been at a very high boiling temperatures, and created that in

a laboratory setup. So they had a primordial soup kind and they had an ocean kind of environment

at a very high temperature, which is the boiling water.

They collected the water vapour under a very reduced environment consisting of methane,

ammonia and hydrogen. Now in this reduced environment, when the energy was supplied,

to mimic ultra-violet radiations or lightnings through electrodes, whatever was being spontaneously

generated was then eventually condensed by the means of a condenser, and then collected

at the collecting pole, or the collecting conical flask. And when they analyze the composition

of this cooled water, they found to their amazement that it consisted of a lot of amino-acids

that we even see today, such as alinine and glycine, sugars, nucleic acid, and nucleic

acid bases like adenine and lipids. So this was one of the earliest proofs to the hypothesis

which was put forth in nineteen twenty-nine, that the early forms of biological molecules

must have had an abiotic synthesis.

But then, just having small molecules is not enough, because if we were to look at the

present day life, you find that you have macromolecules, and not just smaller molecules. For example,

if you look at the plant cell, the outer covering of the plant cell is the cell wall, and well

come to it when were talking about cell structure and cell function is actually a

polymer of glucose, which is made up of, which is what you call as cellulose, and these are

huge polymeric molecules.

So how is it that these abiotic molecules eventually went on to polymerize and form

higher order molecules? So the second hypothesis is that during the early phase of earth, when

the earths crust was still very hot, you find that there were hot rocks, clay or sand,

and under such hot conditions, it was very easy for one monomer to interact with another

monomer, through the process of dehydration. And since there were sufficient carbonaceous

material available in the earths crust, the rate of synthesis was much much higher

than the rate of hydrolysis. So the postulation is that after the abiotic synthesis of monomeric

units, there must have been spontaneous polymerization due to very high rate of dehydration and that

is simply because of the presence of hot rock, clay or sand which will promote this chemical

reaction.

And the proof of this was then supplied by Sydney Fox where he went about dripping actual

organic monomers onto a hot clay. To be specific, on iron pyrite or on sand, which do have these

charged sites, and he found that these monomeric molecules would eventually join together to

form polymeric molecules, and in the process it is the metal ions which are helping or

facilitating

the process of condensation. So, we do know, or we do now believe that yes, the initial

set of biological molecules were from abiotic synthesis, then you ended up having polymerization

of these molecules, and then the question is, ‘When did the replicative ability begin’?

Because all this while, we are just talking about chemical reactions which are happening

in a primordial earth, which has got a highly reducing environment, still doesn't have atmospheric

oxygen, and it still has a very hot surface.

But, as I told you in the initial part of my presentation, what sets apart life from

the non-living things is the ability to replicate. At this time, we dont really have the clear

proof as to how this change happened from a non-replicative to the replicative form

and life, but, if one were to look at which set of molecules would have been the ideal

initiator of this process, we now have sufficient proof or suggestions which suggest that the

earliest genetic material must have been RNA.

Now before I get to why we think it would have been RNA, it's important to understand

the flow of information in a living world. And we all know, and that in our system, any

organism for that matter, most of the information is encoded into what you call as the DNA,

as of today. And this DNA is then transcribed and this process is called as transcription

into RNA, followed by the actual work horses of your body or a cell, which is the protein.

But, if one were to look at RNA.

so we will discuss DNA to RNA, the process of transcription, RNA to protein, the process

of translation, in subsequent videos.

But for today, well focus our attention on RNA. So, there are enough proofs which

suggest that RNA might have been the earliest genetic material, and this was also supported

by the recent findings in nineteen eighties, where Sydney Altman and team, that RNA can

not only polymerize, it is also has the ability to cleave itself. So, it not only acts as

a molecule which can polymerize itself, it also can act as an enzyme, and this catalytic

activity of RNA is not dependent on proteins.

So it's a very versatile nucleic acid molecule in that sense, that it not only can polymerize

itself if the need be, it can cleave, and if the need be, it can also act as an enzyme.

What is even more interesting and intriguing is to note that as we go into the details

of protein synthesis, multiple steps in protein synthesis are dependent on RNA. For example,

the actual machinery which puts this entire chain of amino acids into a protein which

you call as the ribosomes, is chemically made up of RNA. Two third of ribosomes is made

up of RNA. And not just that, even in todays world, where DNA, the so-called our genetic

material which has the entire coded information.

The DNA needs to replicate, it does depend on RNA, and Ill talk about this, when we

talk about the process of DNA replication. So we still dont have concrete proof, but

all these properties of RNA make it a very interesting molecule for it to be probably

the earliest molecule capable of acquiring the ability to replicate.

Then the fourth theory is, ‘how did cells come into existence’? I mean all this while

we are only talking about chemical synthesis of monomers, their polymerization to larger

forms, hopefully acquisition of replicative ability. But then, how do you get to cells?

And, this is where we have to consider that slowly, as the time went past, chances are

the sources became skewed, lesser and lesser.

There was far more entity for these cluster of molecules to kind of guard and maintain

their properties, and in the process what would have happened is that lipid molecules

would have assembled like liposomes. These are amphiphatic, in the sense that they do

have a polar head, and a non-polar tail, and these, it's like you take a soap solution

and when you drop it in water, these lipid molecules will end up forming bubbles. So

these initial lipid liposomes would have somehow enclosed these abiotically synthesized self-replicating

biological molecules to form the first and the most earliest form of life, which is what

you call as the protobionts.

And somewhere down the line, the protobionts would have then evolved into prokaryotes.

But mind you, this time scale of change from abiotic synthesis all the way to prokaryotes

has not happened overnight, it has happened over millions of years, and then, the earliest

form of prokaryotes would have evolved into what we know as eukaryotes, well talk about

this when we talk about the cell biology, and would have finally evolved into what we

see today as multi-

cellular highly evolved organisms. So this journey, all the way from protobionts to the

present day organisms, well cover them in the, in the topic of evolution.

But for now, so you find that there have been four hypotheses, abiotic synthesis of small

molecules, polymerization of small molecules, ability of the small molecules to replicate,

and then, the enclosure of these biological soups, or these biological molecules by means

of lipid molecules, and probably the earliest forms of plasma membrane. But then, there

has been a very important and one of the most crucial turn of events in history of earth,

and that has been the oxygen revolution.

So if you see this curve where I have plotted atmospheric oxygen at the present day, right?

You find that the earlier earth had very low oxygen. Almost for the first two and a half

billion years, you find that there was hardly any oxygen in the atmosphere.

Something must have happened at this turn, which would have led to the sudden burst,

and if you noticed, within a few million years, from hardly any oxygen to reach a point where

the atmosphere becomes highly oxidized. And it's at this stage, we believe in earths

life that the plants or the photosynthetic organisms must have evolved. And the reason

must have been pretty evident, the reason being that by this time, by the time of two

and a half billion years, resources must have become lesser, it would have compelled the

organisms to survive as I said, one property of life is to survive.

So if the resources are getting lower and lower, all that pool of carbonaceous inner

compounds, energy rich phosphates, if they are getting exhausted, the organism, since

it needs to survive, has to find alternate means of obtaining this energy, and one possible

way by which it would have done that would have been to use the reducing power which

it did, as per our understanding in the initial phases using hydrogen sulphide. But then even

hydrogen sulphide would have got exhausted and then, the organism must have developed

a much smarter strategy of actually utilizing the unlimited pool of energy from solar energy

or from the sun and hence the process of photosynthesis evolved.

And in the process of this, oxygen became a by-product and as a result the atmosphere

becomes highly oxidized. Now, if the atmosphere has become highly oxidized, there are still

ways by which the organism has to derive energy by breaking down it's larger molecules into

smaller entities. Earlier it was fairly easy. But now in this oxidized environment, there

had to be a specialized structure possible to efficiently do breakdown of these polymers.

The release of energy, and that is where it is postulated that the mechanism of what you

call today as respiration must have evolved. So in other words, we started the initial

life on earth in a highly reducing environment, and somewhere around I would say close to

about two point six billion years ago, the transition must have happened where the organisms

became smarter, started utilizing the solar energy, and the chemical energy from other

reducing compounds like hydrogen sulphide, and in the process, they started generating

oxygen as a by-product, because of which now the atmosphere became highly oxidized.

So the new set of chemical reactions had to evolve which under these conditions, oxidizing

conditions could still breakdown polymers into monomers for release of energy, and that

is somewhere here probably, that the evolution of mitochondria would have happened.

So lets come back to the slide which I was talking about. So this is where we think

the origin of earth happened. This was the phase, I would say all the way up to here,

you find that the earths atmosphere must have been highly reducing, and then, you have

evidences of early life, somewhere here, which says about seven hundred million years after

the earths origin, the earliest protobionts must have evolved somewhere here, later transitioned

into what you call as the prokaryotes and then due to lack of sufficient high-energy

compounds available and the resources becoming lesser and lesser, a point in evolution would

have happened where the organisms would have developed a property of synthesizing their

own food, using solar energy and in the process, went on generating oxygen as a by-product.

So it is at this point somewhere in earths life that the earths atmosphere became

highly oxidate. And as a process evolved, the prokaryotes ended up becoming well formed,

which is what we call today as eukaryotes, well talk about this, and then somewhere,

as it's life became more and more complex, these single-cell eukaryotes went on to become

multi-cellular eukaryotes, plants, animals, and then finally, the humans.

So, to summarize, what we understand today, is that the origin of life essentially is

divided into three stages; the first stage is the stage of chemical evolution, which

is abiotic formation of organic molecules and it's polymerization; the second is the

self-organization, and the ability of these polymers to somewhere develop the property

of replication and formed the early protobionts.

And then came the actual process from where the life went on evolving again over billions

of years, and these evolutions were in terms of evolutions in metabolic reactions, what

I mean by metabolic reactions are the chemical reactions which happen inside a living organism,

is how we call as metabolism. These metabolic reactions to allow an organism to now synthesize

food, and having synthesized food, also have the ability to break down the food if it needs

to have some energy, and eventually transition from a single-celled organism to a multi-cellular

organism.

One point which still puzzles, and is still very intriguing in life, and study of life

is, that despite the varied forms of life that we see today, what we really find interesting

is that when we look at some of the fundamental chemical reactions in biology, what you call

as metabolic reactions, or the way the information is coded in our DNA, genetic information,

you find that right from prokaryotes all the way till humans, a lot of these informations

and the processes are conserved.

In other words, the codes by which the messages are stored in DNA, the information is stored

in DNA, you find that that language which is used for coding, or information has remained

more or

less uniform all across life, starting from bacteria all the way from human beings, and

yet, they are highly diverse when you look at our forms and our phenotypes; phenotypes

are our physical appearances, and well try to answer some of these questions in our

next class which is on evolution.

So with that, well end origin of life, I would urge you to go through some of the

very informative videos, which very explicitly and beautifully through animation also try

to explain you how life must have evolved on earth. Thank you.

The Description of Origin of Life