#CarlSagan covers a wide range of scientific subjects, including the origin of life and a #perspective of our place in the universe...
A 13-part #documentary #series that covers a wide range of #scientific #subjects, including the #origin of #life and a #perspective of our place in the universe narrated by famous American #Scientist – #Carl #Sagan.
A 13-part #documentary #series that covers a wide range of #scientific #subjects, including the #origin of #life and a #perspective of our place in the universe narrated by famous American #Scientist – #Carl #Sagan.
Category
📚
LearningTranscript
00:00:00Oh
00:01:00All my life I've wondered about life beyond the Earth.
00:01:07On those countless other planets that we think circle other suns, is there also life?
00:01:14Might the beings of other worlds resemble us, or would they be astonishingly different?
00:01:19What would they be made of?
00:01:21In the vast Milky Way galaxy, how common is what we call life?
00:01:30The nature of life on Earth and the quest for life elsewhere are the two sides of the same question.
00:01:37The search for who we are.
00:01:39All living things on Earth are made of organic molecules, a complex microscopic architecture built around atoms of carbon.
00:01:54In the great dark between the stars, there also are organic molecules in immense clouds of gas and dust.
00:02:02And inside such clouds, there are batches of new worlds just forming.
00:02:12Their surfaces are very likely covered with organic molecules.
00:02:16These molecules almost certainly are not made by life, although they are the stuff of life.
00:02:22On suitable worlds, they may lead to life.
00:02:25Organic matter is abundant throughout the cosmos, produced by the same chemistry everywhere.
00:02:32Perhaps given enough time, the origin and evolution of life is inevitable on every clement world.
00:02:55There will surely be some planets too hostile for life.
00:02:59On others, it may arise and die out, or never evolve beyond its simplest forms.
00:03:05And on some small fraction of worlds, there may develop intelligences and civilizations more advanced than ours.
00:03:17All life on our planet is closely related.
00:03:20We have a common organic chemistry and a common evolutionary heritage.
00:03:24And so our biologists are profoundly limited.
00:03:29They study a single biology.
00:03:31One lonely theme in the music of life.
00:03:36Is it the only voice for thousands of light years?
00:03:39Or is there a cosmic fugue?
00:03:42A billion different voices playing the life music of the galaxy.
00:03:47This blue world is where we grew up.
00:03:56There was once a time before life.
00:03:58Our planet is now burgeoning with life.
00:04:02How did it come about?
00:04:03How were organic molecules originally made?
00:04:07How did life evolve to produce beings as elaborate and complex as we?
00:04:13Able to explore the mystery of our own origins.
00:04:16Let me tell you a story about one little phrase in the music of life on Earth.
00:04:25In the history of humans, in the 12th century, Japan was ruled by a clan of warriors called the Heike.
00:04:51The nominal leader of the Heike, the emperor of Japan, was a seven-year-old boy named Antoku.
00:05:09His guardian was his grandmother, the lady Ni'i.
00:05:12The Heike were engaged in a long and bloody war with another samurai clan, the Genji.
00:05:29Each asserted a superior ancestral claim to the imperial throne.
00:05:39Their decisive encounter occurred at Tanuura, in the Japanese inland sea, on April 24th, in the year 1185.
00:05:52The Heike were badly outnumbered and outmaneuvered.
00:05:59With their cause clearly lost, the surviving Heike warriors threw themselves into the sea and drowned.
00:06:06The emperor's grandmother, the lady Ni'i, resolved that they would not be captured by the enemy.
00:06:18What happened next is related in the tale of the Heike.
00:06:22The young emperor asked the lady Ni'i, where are you to take me?
00:06:28She turned to the youthful sovereign, with tears streaming down her cheeks, and comforted him.
00:06:34Blinded with tears, the child sovereign put his beautiful small hands together.
00:06:54He turned first to the east, to say farewell to the god of Issei.
00:07:00And then to the west, to recite the Nembutsu, a prayer to the Amida Buddha.
00:07:07The lady Ni'i took him in her arms, and with the words,
00:07:11In the depths of the ocean is our capital, sank with him at last beneath the waves.
00:07:30The destruction of the Heike battle fleets at Dano Ura marked the end of the clan's 30-year rule.
00:07:47The Heike all but vanished from history.
00:07:49Only 43 Heike survived, all women.
00:07:58These former ladies-in-waiting of the imperial court were reduced to selling flowers and other favors
00:08:05to the fishermen near the scene of the battle.
00:08:08These women, and their offspring by the fisher-folk, established a festival to commemorate the battle.
00:08:28To this day, every year, on the 24th of April, their descendants proceed to the Akama Shrine,
00:08:35which contains the mausoleum of the drowned, seven-year-old emperor, Antoku.
00:08:45There, they conduct a ceremony of remembrance for the life and death of the Heike warriors.
00:08:52But there's a strange postscript to this story.
00:09:04The fishermen say that the Heike samurai wander the bottom of the inland sea still,
00:09:11in the form of crabs.
00:09:14There are crabs to be found here, which have curious markings on their backs,
00:09:18patterns which resemble a human face,
00:09:22with the aggressive scowl of a samurai warrior from medieval Japan.
00:09:35These Heike crabs, when caught, are not eaten.
00:09:38They're thrown back into the sea,
00:09:41in commemoration of the doleful events,
00:09:44of the Battle of Dano Ura.
00:09:46This legend raises a lovely problem.
00:10:01How does it come about
00:10:02that the face of a warrior
00:10:05is cut on the carapace
00:10:07of a Japanese crab?
00:10:09How could it be?
00:10:10The answer seems to be
00:10:12that humans made this face.
00:10:14But how?
00:10:17Like many other features,
00:10:18the patterns on the back or carapace of this crab
00:10:22are inherited.
00:10:24But among crabs, as among humans,
00:10:26there are many different hereditary lines.
00:10:29Now, suppose purely by chance,
00:10:32among the distant ancestors of this crab,
00:10:36they came to be one which looked
00:10:37just a little bit like a human face.
00:10:40Long before the battle,
00:10:42fishermen may have been reluctant
00:10:43to eat a crab with a human face.
00:10:47In throwing it back into the sea,
00:10:49they were setting into motion
00:10:50a process of selection.
00:10:53If you're a crab,
00:10:55and your carapace is just ordinary,
00:10:58the humans are going to eat you.
00:11:00But if it looks a little bit like a face,
00:11:03they'll throw you back,
00:11:04and you'll be able to have lots of nice little baby crabs
00:11:07that all look just like you.
00:11:09As many generations passed,
00:11:11of crabs and fisher folk alike,
00:11:14the crabs with patterns
00:11:16that look most like a samurai face
00:11:18preferentially survived.
00:11:21Until eventually,
00:11:23there was produced not just a human face,
00:11:25not just a Japanese face,
00:11:27but the face of a samurai warrior.
00:11:29All this has nothing to do
00:11:32with what the crabs might want.
00:11:35Selection is imposed from the outside.
00:11:38The more you look like a samurai,
00:11:40the better your chances of survival.
00:11:43Eventually, there are a lot of crabs
00:11:45that look like samurai warriors.
00:11:47This process is called artificial selection.
00:12:13In the case of the Heikki crab,
00:12:14it was affected more or less unconsciously
00:12:17by the fishermen,
00:12:18and certainly without any serious contemplation
00:12:21by the crabs.
00:12:23Humans, for thousands of years,
00:12:26have deliberately selected
00:12:27which plants and animals shall live.
00:12:30We're surrounded with farm and domestic animals,
00:12:34fruits, vegetables.
00:12:36Where did they come from?
00:12:37Were they once pre-living
00:12:39in their present form in the wild,
00:12:40and then induced to adopt
00:12:41some less strenuous life on the farm?
00:12:45No.
00:12:46They are, almost all of them,
00:12:48made by us.
00:12:53The essence of artificial selection
00:12:55for a horse or a cow,
00:12:58a grain of rice or a Heikki crab,
00:13:00is this.
00:13:02Many characteristics are inherited.
00:13:03They breed true.
00:13:07Humans encourage the reproduction
00:13:09of some varieties
00:13:10and discourage the reproduction of others.
00:13:13The variety selected for
00:13:15eventually becomes abundant.
00:13:16The variety selected against
00:13:18becomes rare,
00:13:19maybe extinct.
00:13:22But if artificial selection
00:13:23makes such changes
00:13:25in only a few thousand years,
00:13:27what must natural selection,
00:13:30working for billions of years,
00:13:32be capable of?
00:13:34The answer is all the beauty
00:13:36and diversity
00:13:37in the biological world.
00:13:43That life evolved
00:13:45over the ages
00:13:46is clear
00:13:47from the changes we've made
00:13:49in the beasts and vegetables,
00:13:51but also
00:13:52from the record
00:13:54in the rocks.
00:13:56The fossil evidence
00:13:57speaks to us
00:13:58unambiguously
00:13:59of creatures
00:13:59that were once present
00:14:01in enormous numbers
00:14:03and that have now
00:14:04vanished utterly.
00:14:06There are far more species
00:14:07that have become extinct
00:14:08than exist today.
00:14:10They're the
00:14:10terminated experiments
00:14:12in evolution.
00:14:15These guys,
00:14:15for example,
00:14:16the trilobites,
00:14:17appeared 600 million years ago.
00:14:19They were around
00:14:20for 300 million years.
00:14:22They're all gone.
00:14:24There's none left.
00:14:26But in those old rocks,
00:14:27there are no fossils
00:14:28of people
00:14:29or cattle.
00:14:31We've evolved
00:14:31only recently.
00:14:34Evolution
00:14:34is a fact,
00:14:36not a theory.
00:14:37It merely happened.
00:14:45That the mechanism
00:14:46of evolution
00:14:47is natural selection
00:14:48was the great discovery
00:14:49of Charles Darwin
00:14:50and Alfred Russell Wallace.
00:14:52Here's how it works.
00:14:55Nature is prolific.
00:14:57There are many more
00:14:58creatures that are born
00:14:59than can possibly survive.
00:15:01So those varieties,
00:15:03which are by accident
00:15:04less well adapted,
00:15:06don't survive.
00:15:07Or at least
00:15:08they leave fewer offspring.
00:15:10Now, mutations,
00:15:12sudden changes in heredity,
00:15:14are passed on.
00:15:15They breed true.
00:15:17The environment
00:15:18selects those
00:15:19occasional mutations
00:15:20which enhance survival.
00:15:21and the resulting
00:15:23series of slow changes
00:15:25in the nature
00:15:25of living beings
00:15:26is the origin
00:15:27of new species.
00:15:29Well,
00:15:30many people
00:15:31were scandalized
00:15:33by the ideas
00:15:34of evolution
00:15:35and natural selection.
00:15:37Our ancestors
00:15:37looked at
00:15:38the intricacy
00:15:39and beauty
00:15:40of life
00:15:41and saw
00:15:42evidence
00:15:43for a great
00:15:44designer.
00:15:45the simplest organism
00:15:50is a far more
00:15:51complex machine
00:15:52than the finest
00:15:53pocket watch.
00:15:55And yet,
00:15:55pocket watches
00:15:56don't spontaneously
00:15:58self-assemble
00:15:59or evolve
00:16:00in slow stages
00:16:02on their own
00:16:03from, say,
00:16:03grandfather clocks.
00:16:06A watch
00:16:06implies a watchmaker.
00:16:08So,
00:16:09there seemed
00:16:11to be no way
00:16:11in which atoms
00:16:12could spontaneously
00:16:14fall together
00:16:15and create,
00:16:16say,
00:16:17a dandelion.
00:16:20The idea
00:16:22of a designer
00:16:23is an appealing
00:16:25and altogether
00:16:25human explanation
00:16:26of the biological world.
00:16:28But,
00:16:29as Darwin
00:16:30and Wallace
00:16:31showed,
00:16:32there's another way.
00:16:33Equally human
00:16:34far more
00:16:36complex.
00:16:37Natural selection
00:16:39which makes
00:16:40the music of life
00:16:41more beautiful
00:16:41as the eons
00:16:43pass.
00:16:50To understand
00:16:51the passage
00:16:52of the eons,
00:16:53we've compressed
00:16:53all of time
00:16:54into a single
00:16:55cosmic year
00:16:56with the Big Bang
00:16:57on January 1st.
00:17:00Every month
00:17:00here represents
00:17:01a little over
00:17:02a billion years.
00:17:04The Earth
00:17:05didn't form
00:17:05until the
00:17:06cosmic year
00:17:06was two-thirds
00:17:07over.
00:17:08Our understanding
00:17:09of the history
00:17:10of life
00:17:11is very recent,
00:17:13occupying only
00:17:14the last few
00:17:14seconds of
00:17:15December 31st,
00:17:16that small
00:17:17white spot
00:17:18at bottom right
00:17:18in the cosmic
00:17:19calendar.
00:17:21What happened
00:17:21on Earth
00:17:22may be more
00:17:22or less
00:17:23typical of
00:17:24the evolution
00:17:24of life
00:17:24on many worlds,
00:17:26but in its
00:17:27details,
00:17:28the story
00:17:29of life
00:17:29on Earth
00:17:29is probably
00:17:30unique in all
00:17:32the Milky Way
00:17:32galaxy.
00:17:33The secrets
00:17:35of evolution
00:17:35are time
00:17:37and death.
00:17:39Time for the
00:17:40slow accumulation
00:17:41of favorable
00:17:41mutations
00:17:42and death
00:17:43to make room
00:17:45for new
00:17:45species.
00:17:47Life on Earth
00:17:48arose in September
00:17:49of the cosmic
00:17:49calendar
00:17:50when our world,
00:17:52still heavily
00:17:52battered and
00:17:53cratered from
00:17:54its violent
00:17:54origin,
00:17:55may have looked
00:17:56a little like
00:17:56the moon.
00:17:57The Earth
00:18:00is about
00:18:01four and a half
00:18:02billion years
00:18:03old.
00:18:03In the
00:18:04cosmic calendar,
00:18:05it's condensed
00:18:06out of
00:18:07interstellar gas
00:18:08and dust
00:18:09around
00:18:10September 14th.
00:18:12We know
00:18:12from the fossil
00:18:13record that
00:18:14the origin
00:18:14of life
00:18:14happened
00:18:15soon afterwards,
00:18:16maybe around
00:18:17September 25th,
00:18:19something like
00:18:19that,
00:18:19probably in the
00:18:20ponds and oceans
00:18:21of the primitive
00:18:22Earth.
00:18:23Now,
00:18:24the first
00:18:24living things
00:18:25were not
00:18:25anything so
00:18:26complex as
00:18:27a one-celled
00:18:28organism,
00:18:28which already
00:18:29is a highly
00:18:30sophisticated
00:18:31form of life.
00:18:32No,
00:18:33the first
00:18:33stirrings of
00:18:34life were
00:18:35much more
00:18:35humble and
00:18:37happened on
00:18:38the molecular
00:18:38level.
00:18:39In those early
00:18:40days,
00:18:41lightning and
00:18:42ultraviolet light
00:18:43from the sun
00:18:43were breaking
00:18:44apart simple
00:18:45hydrogen-rich
00:18:46molecules in
00:18:47the primitive
00:18:47atmosphere,
00:18:48and the fragments
00:18:49of the molecules
00:18:50were spontaneously
00:18:51recombining
00:18:52into more
00:18:54and more
00:18:54complex
00:18:55molecules.
00:18:58The products
00:19:00of this early
00:19:00chemistry
00:19:01dissolved in
00:19:02the oceans,
00:19:03forming a
00:19:04kind of
00:19:04organic soup
00:19:06of gradually
00:19:07increasing
00:19:08complexity,
00:19:09until one
00:19:10day,
00:19:11quite by
00:19:12accident,
00:19:13a molecule
00:19:14arose which
00:19:14was able to
00:19:15make crude
00:19:15copies of
00:19:16itself,
00:19:17using as
00:19:18building blocks
00:19:18the other
00:19:19molecules in
00:19:20the soup.
00:19:21This was
00:19:21the ancestor
00:19:22of DNA,
00:19:24the master
00:19:26molecule of
00:19:27life on
00:19:27Earth.
00:19:28It's made of
00:19:29four different
00:19:29molecular parts
00:19:30called nucleotides,
00:19:32which constitute
00:19:33the four letters
00:19:34of the genetic
00:19:35code, the language
00:19:36of heredity.
00:19:38Each of the four
00:19:38nucleotides, the
00:19:40rungs on the DNA
00:19:42ladder, are a
00:19:43different color in
00:19:44this model.
00:19:45The instructions
00:19:46that are spelled
00:19:47out are different
00:19:48for different
00:19:48organisms.
00:19:49That's why
00:19:49organisms are
00:19:50different.
00:19:51Now, a
00:19:52mutation is a
00:19:54change of a
00:19:55nucleotide, a
00:19:56misspelling of
00:19:57the genetic
00:19:57instructions.
00:19:59Most mutations
00:19:59spell genetic
00:20:00nonsense, as
00:20:01you'd expect,
00:20:02because they're
00:20:02random.
00:20:04They're harmful
00:20:04for the next
00:20:05generation.
00:20:06But a very
00:20:07few, by
00:20:08accident, make
00:20:09better sense
00:20:09than the original
00:20:10instructions and
00:20:11aid the evolution
00:20:12of life.
00:20:14DNA is about
00:20:16a billion times
00:20:17smaller than we
00:20:18see it here.
00:20:19In fact, each
00:20:20of these things
00:20:21that looks like a
00:20:21piece of fruit
00:20:22is an atom.
00:20:24Without the
00:20:25tools of science,
00:20:26the machinery of
00:20:27life would be
00:20:28invisible.
00:20:33Four billion
00:20:35years ago, the
00:20:36ancestors of DNA
00:20:37were competing for
00:20:38molecular building
00:20:39blocks and leaving
00:20:40crude copies of
00:20:41themselves.
00:20:43There were no
00:20:43predators.
00:20:44The stuff of life
00:20:45was everywhere, so
00:20:47the oceans and
00:20:47the murky pools
00:20:48that filled the
00:20:49craters were, for
00:20:50these molecules, a
00:20:52garden of Eden.
00:20:53With reproduction,
00:20:55mutation, and
00:20:55natural selection, the
00:20:57evolution of living
00:20:58molecules was well
00:21:00underway.
00:21:01Varieties with
00:21:02specialized functions
00:21:03then joined together,
00:21:06making a collective,
00:21:07the first cell.
00:21:09The organic soup
00:21:10eventually ate itself
00:21:11up.
00:21:12But by this time,
00:21:13plants had evolved,
00:21:15able to use sunlight
00:21:15to make their own
00:21:16building blocks.
00:21:18They turned the
00:21:19waters green.
00:21:20A number of one-cell
00:21:21plants joined together,
00:21:23the first multicellular
00:21:24organisms.
00:21:27Equally important was
00:21:29the invention, not made
00:21:31until early November,
00:21:32of sex.
00:21:33It was stumbled upon
00:21:34by the microbes.
00:21:35By December 1st,
00:21:41green plants had
00:21:41released copious amounts
00:21:43of oxygen and nitrogen
00:21:44into the atmosphere.
00:21:47The sky is made by life.
00:21:51Then, suddenly, on
00:21:53December 15th, there was
00:21:54an enormous proliferation
00:21:56of new life forms, an
00:21:57event called the
00:21:59Cambrian Explosion.
00:22:00We know from the
00:22:06fossil record that life
00:22:08arose shortly after
00:22:09the formation of the
00:22:10Earth, suggesting that
00:22:12the origin of life
00:22:13might be an inevitable
00:22:15chemical process on
00:22:17countless Earth-like
00:22:18planets throughout the
00:22:19cosmos.
00:22:20But on the Earth,
00:22:21in nearly 4 billion
00:22:23years, life advanced
00:22:24no further than algae.
00:22:27So maybe more complex
00:22:29forms of life are
00:22:30harder to evolve, harder
00:22:32even than the origin
00:22:33of life itself.
00:22:34If this is right, the
00:22:36planets of the galaxy
00:22:37might be filled with
00:22:39microorganisms, but big
00:22:41beasts and vegetables
00:22:42and thinking beings
00:22:44might be comparatively
00:22:46rare.
00:22:50By December 18th, there
00:22:52were vast herds of
00:22:54trilobites foraging on
00:22:55the ocean bottom, and
00:22:57squid-like creatures with
00:22:59their multicolored
00:23:00shells were everywhere.
00:23:05We know enough to
00:23:06sketch in a few of the
00:23:07subsequent details.
00:23:09The first fish and the
00:23:10first vertebrates appeared
00:23:11on December 19th.
00:23:13Plants began to
00:23:14colonize the land on
00:23:15December 20th.
00:23:16The first winged insects
00:23:18fluttered by on
00:23:19December 22nd.
00:23:21And on this date also,
00:23:22there were the first
00:23:23amphibians, creatures
00:23:25something like the
00:23:26lungfish, able to
00:23:27survive both on land
00:23:29and in water.
00:23:30Our direct ancestors
00:23:32were now leaving the
00:23:33oceans behind.
00:23:37The first trees and the
00:23:39first reptiles evolved on
00:23:41December 23rd.
00:23:43Two amazing evolutionary
00:23:44evolutionary developments.
00:23:48We're descended from some
00:23:50of those reptiles.
00:23:55The dinosaurs appeared on
00:23:58Christmas Eve.
00:23:59There were many different
00:24:00kinds of dinosaurs.
00:24:02The Earth was once their
00:24:04planet.
00:24:04Many stood upright and had
00:24:10some fair intelligence.
00:24:12Great lizards crashed and
00:24:14thundered through the
00:24:15steaming jungles.
00:24:22Unnoticed by the dinosaurs,
00:24:23a new creature, whose young
00:24:25were born live and helpless,
00:24:27was making its timid debut.
00:24:29The first mammals emerged
00:24:31on December 26th.
00:24:32The first birds on the
00:24:35following day.
00:24:39But the dinosaurs still
00:24:41dominated the planet.
00:24:43Then, suddenly, without
00:24:45warning, all over the
00:24:46planet at once, the
00:24:47dinosaurs died.
00:24:49The cause is unknown, but
00:24:51the lesson is clear.
00:24:52Even 160 million years on a
00:24:54planet is no guarantee of
00:24:56survival.
00:24:57The dinosaurs perished around
00:24:59the time of the first
00:25:01flower.
00:25:02On December 30th, the first
00:25:05creatures who looked even a
00:25:07little bit human evolved,
00:25:09accompanied by a spectacular
00:25:11increase in the size of their
00:25:12brains.
00:25:14And then, on the evening of the
00:25:16last day of the last month,
00:25:18only a few million years ago,
00:25:19the first true humans took
00:25:22their place on the cosmic
00:25:23calendar.
00:25:24The written record of human
00:25:27history occupies only the last
00:25:29ten seconds of the cosmic
00:25:30year.
00:25:34Now, let's take a closer look at
00:25:36who our ancestors were.
00:25:38A simple chemical circumstance
00:25:40led to one of the great
00:25:41moments in the history of our
00:25:42planet.
00:25:43There were many kinds of
00:25:44molecules in the primordial soup.
00:25:47Some were attracted to water on
00:25:48one side and repelled by it on the
00:25:51other.
00:25:52This drove them together into a
00:25:55tiny, enclosed, spherical shell,
00:25:57like a soap bubble, which
00:25:59protected the interior.
00:26:00Within the bubble, the
00:26:02ancestors of DNA found a home,
00:26:04and the first cell arose.
00:26:06It took hundreds of millions of
00:26:08years for tiny plants to evolve,
00:26:10giving off oxygen.
00:26:12But that branch didn't lead to us.
00:26:14bacteria that could breathe
00:26:17oxygen took over a billion more
00:26:19years to evolve.
00:26:21From a naked nucleus, a cell
00:26:24developed with a nucleus inside.
00:26:27Some of these amoeba-like forms
00:26:30led eventually to plants.
00:26:36Others produced colonies with
00:26:38inside and outside cells
00:26:40performing different functions.
00:26:41becoming a polyp attached to the
00:26:48ocean floor, filtering food from
00:26:51the water, and evolving little
00:26:54tentacles to direct food into a
00:26:57primitive mouth.
00:26:59This humble ancestor of ours also
00:27:02led to spiny-skinned, armored
00:27:04animals with internal organs, including
00:27:06our cousin, the starfish.
00:27:09But we don't come from starfish.
00:27:13About 550 million years ago,
00:27:16filter feeders evolved gill slits,
00:27:19which were more efficient at
00:27:20straining food particles from the
00:27:22water.
00:27:23One evolutionary branch led to
00:27:24acorn worms.
00:27:27Another led to a creature which
00:27:29swam freely in the larval stage,
00:27:31but as an adult was still firmly
00:27:33anchored to the ocean floor.
00:27:35Some became living hollow tubes.
00:27:37But others retained the larval forms
00:27:41throughout the life cycle and became
00:27:43free-swimming adults with something like a backbone.
00:27:51Our ancestors now, 500 million years ago,
00:27:55were jawless, filter-feeding fish,
00:27:58a little like lamp rays.
00:28:00Gradually, those tiny fish evolved,
00:28:05eyes and jaws.
00:28:08Fish then began to eat one another.
00:28:10If you could swim fast, you survived.
00:28:15If you had jaws to eat with,
00:28:17you could now use your gills to breathe the oxygen in the water.
00:28:20This is the way modern fish arose.
00:28:22During the summer,
00:28:28some swamps and lakes dried up,
00:28:30so some fish evolved a primitive lung
00:28:32to breathe air until the rains came.
00:28:35Their brains were getting bigger.
00:28:38If the rains didn't come,
00:28:39it was handy to be able to pull yourself
00:28:41along to the next swamp.
00:28:43That was a very important adaptation.
00:28:45The first amphibians evolved,
00:28:51still with a fish-like tail.
00:28:53Amphibians, like fish,
00:28:55laid their eggs in water
00:28:56where they were easily eaten.
00:28:57But then a splendid new invention came along,
00:29:00the hard-shelled egg laid on the land
00:29:03where there were as yet no predators.
00:29:05Reptiles and turtles go back to those days.
00:29:11Many of the reptiles hatched on land
00:29:13and never returned to the waters.
00:29:15Some became the dinosaurs.
00:29:18One line of dinosaurs developed feathers
00:29:20useful for short flights.
00:29:23Today, the only living descendants of the dinosaurs
00:29:25are the birds.
00:29:29The great dinosaurs evolved along another branch.
00:29:32Some were the largest flesh-eaters ever to walk the land.
00:29:35But 65 million years ago,
00:29:37they all mysteriously perished.
00:29:41Meanwhile,
00:29:41the forerunners of the dinosaurs
00:29:43were also evolving in a different direction.
00:29:46Small, scurrying creatures,
00:29:49with the young growing inside the mother's body.
00:29:52After the extinction of the dinosaurs,
00:29:54many different forms developed.
00:30:01The young were very immature at birth.
00:30:04In the marsupials,
00:30:05the wombat, for example,
00:30:07and in the mammals,
00:30:08the young had to be taught how to survive.
00:30:11The brain grew larger still.
00:30:13Something like a shrew
00:30:15was the ancestor of all the mammals.
00:30:22One line took to the trees,
00:30:25developing dexterity,
00:30:27stereo vision,
00:30:28larger brains,
00:30:29and a curiosity about their environment.
00:30:31Some became baboons.
00:30:33But that's not the line to us.
00:30:38Apes and humans have a recent common ancestor.
00:30:41Bone for bone,
00:30:42muscle for muscle,
00:30:43molecule for molecule.
00:30:45There are almost no important differences
00:30:47between apes and humans.
00:30:49And the chimpanzee,
00:30:55our ancestors walked upright,
00:30:57freeing their hands
00:30:59to poke and fix an experiment.
00:31:02We got smarter.
00:31:03We began to talk.
00:31:11Many collateral branches of the human family
00:31:14became extinct in the last few million years.
00:31:17We, with our brains in our hands,
00:31:21are the survivors.
00:31:23There's an unbroken thread
00:31:24that stretches from those first cells to us.
00:31:28Let's look at it again,
00:31:30compressing 4 billion years of evolution
00:31:32into 40 seconds.
00:32:03Those are some of the things that molecules do,
00:32:23given four billion years of evolution.
00:32:27We sometimes represent evolution as the ever-branching ramifications
00:32:32of some original trunk,
00:32:34each branch pruned and clipped by natural selection.
00:32:38Every plant and animal alive today
00:32:41has a history as ancient and illustrious as ours.
00:32:45Humans stand on one branch,
00:32:49but now we affect the future of every branch
00:32:52of this four-billion-year-old tree.
00:32:58How lovely trees are.
00:33:01The human species grew up in and around them.
00:33:05We have a natural affinity for trees.
00:33:08Trees photosynthesize.
00:33:10They harvest sunlight.
00:33:13They compete for the sun's favors.
00:33:17Look at those two trees over there,
00:33:20pushing and shoving for sunlight,
00:33:23but with grace and astonishing slowness.
00:33:36There are so many plants on the earth
00:33:38that there's a danger of thinking them trivial,
00:33:41of losing sight of the subtlety and efficiency of their design.
00:33:45They are great and beautiful machines,
00:33:48powered by sunlight,
00:33:50taking in water from the ground
00:33:52and carbon dioxide from the air
00:33:54and converting them into food for their use and ours.
00:33:58This is a museum of living plants,
00:34:10the Royal Botanic Gardens at Kew in London.
00:34:13Every plant uses the carbohydrates it makes as an energy source
00:34:25to go about its planty business.
00:34:28And we animals who are ultimately parasites in the plants,
00:34:32we steal the carbohydrates so we can go about our business.
00:34:41In eating the plants and their fruits,
00:34:44we combine the carbohydrates with oxygen,
00:34:47which as a result of breathing we've dissolved in our blood.
00:34:50From this chemical reaction,
00:34:52we extract the energy which makes us go.
00:34:55In the process we exhale carbon dioxide into the atmosphere,
00:34:59which the plants then use to make more carbohydrates.
00:35:04What a marvelous cooperative arrangement.
00:35:07Plants and animals each using the other's waste gases.
00:35:11The whole cycle powered by abundant sunlight.
00:35:15But there would be carbon dioxide in the air,
00:35:18even if there were no animals.
00:35:20We need the plants much more than they need us.
00:35:24There are many family resemblances among the organisms of the Earth.
00:35:31Some are very apparent, such as the use of the number five.
00:35:36Humans have five major bodily projections.
00:35:39One head, two arms, two legs.
00:35:42So do ducks.
00:35:44Although the functions of their bodily projections
00:35:47are not quite the same.
00:35:49An octopus or a centipede has a different plan.
00:35:52And a being from another planet might be much stranger still.
00:35:57These family resemblances continue, and on a much deeper level,
00:36:03when we go to the molecular basis of life.
00:36:05There are tens of billions of different kinds of organic molecules.
00:36:11Yet only about fifty of them are used in the essential machinery of life.
00:36:17The same fifty employed over and over again, ingeniously, for different functions,
00:36:23in every living thing.
00:36:25And when we go to the very kernel of life on Earth,
00:36:28to the proteins that control cell chemistry,
00:36:31to the spiral or helix of the nucleic acids,
00:36:36which carry the hereditary information,
00:36:39we find these molecules to be absolutely identical
00:36:43in all plants and animals of our planet.
00:36:46This oak tree and me, we're made of the same stuff.
00:37:03If you go back far enough, you'll find that we have a common ancestor.
00:37:19That's why our chemistry is so alike.
00:37:22Let's take a trip to examine this common basis of life,
00:37:30a voyage to investigate the molecular machinery at the heart of life on Earth,
00:37:36a journey to the nucleus of the cell.
00:37:39First, we need a cell.
00:37:41I have trillions. I can afford to donate a few.
00:37:52The casual act of pricking a finger is an event of some magnitude
00:37:57on the scale of the very small.
00:37:59Millions of red blood cells are detoured from their usual routes.
00:38:05But most continue to cruise about the body,
00:38:08carrying their cargos of oxygen to the remotest freckle.
00:38:13We're about to enter the living cell,
00:38:16a realm in its own way as complex and beautiful
00:38:19as the realm of galaxies and stars.
00:38:22Among the many red blood cells, we encounter a white blood cell,
00:38:26a lymphocyte, whose job it is to protect me against invading microbes.
00:38:31It makes antibodies on its furrowed surface,
00:38:34but its interior is like that of many cells.
00:38:40Plunging through the membrane, we find ourselves inside the cell.
00:38:44Here, every structure has its function.
00:38:51Those dark green blobs are factories,
00:38:54where messenger molecules are busy building the enzymes
00:38:57which control the chemistry of the cell.
00:38:59The messengers were instructed and dispatched from within the nucleus,
00:39:04the heart and brain of the cell.
00:39:06All the instructions on how to get a cell to work
00:39:09and how to make another are hidden away in there.
00:39:12We find a tunnel, a nuclear pore,
00:39:15an approach to the biological holy of holies.
00:39:18These necklaces, these intricately looped and coiled strands,
00:39:27are nucleic acids, DNA.
00:39:31Everything you need to know on how to make a human being
00:39:34is encoded in the language of life, in the DNA molecule.
00:39:46This is the DNA double helix, a machine with about a hundred billion moving parts, called atoms.
00:39:55There are as many atoms in one molecule of DNA as there are stars in a typical galaxy.
00:40:02The sequence of nucleotides, here brightly colored,
00:40:10is all that's passed on from generation to generation.
00:40:13Change the order of the nucleotides, and you change the genetic instructions.
00:40:18DNA must replicate itself with extreme fidelity.
00:40:30The reproduction of a DNA molecule begins by separating the two helices.
00:40:35This is accomplished by an unwinding enzyme.
00:40:39Like some precision tool, this enzyme, shown in blue,
00:40:43breaks the chemical bonds that connect the nucleotides
00:40:46and bind the two helices of DNA together.
00:40:49The enzyme works its way down the molecule, unzipping DNA as it goes.
00:40:58Each helix copies the other, supervised by special enzymes.
00:41:03The organic soup inside the nucleus contains many free nucleotides.
00:41:07The enzyme recognizes an approaching nucleotide and clicks it into place,
00:41:13reproducing another rung in the double helix.
00:41:20When the DNA is replicating in one of your cells,
00:41:23a few dozen nucleotides are added every second.
00:41:26Thousands of these enzymes may be working on a given DNA molecule.
00:41:30When an arriving nucleotide doesn't fit, the enzyme throws it away.
00:41:46We call this proofreading.
00:41:48On the rare occasions of a proofreading error, the wrong nucleotide is attached,
00:41:52and a small random change has been made in the genetic instructions.
00:41:57A mutation has occurred.
00:42:00This enzyme is a pretty small molecule, but it catches nucleotides,
00:42:05it assembles them in the right order, it knows how to proofread.
00:42:08It's responsible in the most fundamental way for the reproduction of every cell
00:42:13and every being on Earth.
00:42:22That enzyme and DNA itself are molecular machines with awesome powers.
00:42:31Within every living thing, the molecular machines are busy,
00:42:34making sure that nucleic acids will continue to reproduce.
00:43:00A minor cut in my skin sounds a local alarm,
00:43:04and the blood spins a complex net of strong fibers
00:43:08to form a clot and staunch the flow of blood.
00:43:11There's a very delicate balance here.
00:43:13Too much clotting, and your bloodstream will solidify.
00:43:16Too little clotting, and you'll bleed to death from the merest scratch.
00:43:21The balance is controlled by enzymes, instructed by DNA.
00:43:26Down here, there's also a kind of sanitation squad,
00:43:31comprised of white blood cells that swings into action,
00:43:35surrounds invading bacteria, and ravenously consumes them.
00:43:40This mopping up operation is another part of the healing process,
00:43:43again controlled by DNA.
00:43:46These cells are parts of us, but how alien they seem.
00:43:52Within each of them, within every cell,
00:43:55there are exquisitely evolved molecular machines,
00:43:58nucleic acids, enzymes, the cell architecture.
00:44:02Every cell is a triumph of natural selection.
00:44:06And we're made of trillions of cells.
00:44:09We are, each of us, a multitude.
00:44:12Within us is a little universe.
00:44:16Human DNA is a coiled ladder.
00:44:34A billion nucleotides long.
00:44:41Many possible combinations of nucleotides are nonsense.
00:44:45That is, they translate into proteins which serve no useful function whatever.
00:44:50Only a comparatively few nucleic acid molecules are any good for life forms as complicated as we are.
00:44:59But even so, the number of useful ways of assembling nucleic acids is stupefyingly large.
00:45:06It's probably larger than the total number of atoms in the universe.
00:45:11This means that the number of possible kinds of human beings
00:45:16is vastly greater than the number of human beings that has ever lived.
00:45:21This untapped potential of the human species is immense.
00:45:25There must be ways of putting nucleic acids together which will function far better by any criterion you wish to choose
00:45:32than the hereditary instructions of any human being who has ever lived.
00:45:38Fortunately, we do not know, or at least do not yet know,
00:45:43how to assemble alternative sequences of nucleotides to make alternative kinds of human beings.
00:45:51But in the future we might well be able to put nucleotides together in any desired sequence
00:45:57to produce whatever human characteristics we think desirable.
00:46:00A disquieting and awesome prospect.
00:46:05We human beings don't look very much like a tree.
00:46:20We certainly view the world differently than a tree does.
00:46:24But down deep, at the molecular heart of life, we're essentially identical to trees.
00:46:32We both use nucleic acids as the hereditary material.
00:46:37We both use proteins as enzymes to control the chemistry of the cell.
00:46:42And most significantly, we both use the identical code book to translate nucleic acid information into protein information.
00:46:53Any tree could read my genetic code.
00:46:57How did such astonishing similarities come about?
00:47:02Why are we cousins to the trees?
00:47:06Would life on some other planet use proteins, the same proteins, the same nucleic acids, the same genetic code?
00:47:14The usual explanation is that we are, all of us, trees and people, anglerfish, slime molds, bacteria, all descended from a single and common instance of the origin of life four billion years ago in the early days of our planet.
00:47:36Now, how did the molecules of life arise?
00:48:01In a laboratory at Cornell University, we mix together the gases and waters of the primitive Earth, supply some energy, and see if we can make the stuff of life.
00:48:25But what was the early atmosphere made of? Ordinary air?
00:48:29If we start with our present atmosphere, the experiment is a dismal failure.
00:48:34Instead of making proteins and nucleic acids, all we make is smog, a backward step.
00:48:40Why doesn't such an experiment work?
00:48:43Because the air of today contains molecular oxygen.
00:48:46But oxygen is made by plants.
00:48:49It's pretty obvious that there were no plants before the origin of life.
00:48:53We mustn't use oxygen in our experiments because there wasn't any oxygen in the early atmosphere.
00:49:04This is perfectly reasonable because the cosmos is made mostly of hydrogen, which gobbles oxygen up.
00:49:10The Earth's low gravity has allowed most of our original hydrogen gas to trickle away to space.
00:49:16There's almost none left.
00:49:19But four billion years ago, our atmosphere was full of hydrogen-rich gases.
00:49:24Methane, ammonia, water vapor.
00:49:26These are the gases we should use.
00:49:34Taking great care to ensure the purity of these gases, my colleague, Bishan Kari, pumps them from their holding flasks.
00:49:41An experiment like this was first performed by Stanley Miller and Harold Urie in the 1950s.
00:50:01The starting gases are now introduced into a large reaction vessel.
00:50:22We could shine ultraviolet light on this mixture, simulating the early sun.
00:50:26But in this experiment, the gases will be sparked, as the primitive atmosphere was, by early lightning.
00:50:56After only a few hours, the interior of the reaction vessel becomes streaked with a strange brown pigment.
00:51:04A rich collection of complex organic molecules, including the building blocks of the proteins and the nucleic acids.
00:51:15Under the right conditions, these building blocks assemble themselves into molecules resembling little proteins and little nucleic acids.
00:51:22These nucleic acids can even make identical copies of themselves.
00:51:33In this vessel are the notes of the music of life.
00:51:37Although not yet the music itself.
00:51:39Now, no one so far has mixed together the gases and waters of the primitive Earth and at the end of the experiment had something crawl out of the flask.
00:51:54There's still a great deal to be understood about the origin of life, including the origin of the genetic code.
00:52:01But we've only been at such experiments for 30 years.
00:52:05Nature has had a 4 billion year head start.
00:52:08Incidentally, there's nothing in such experiments that's unique to the Earth.
00:52:13The gases we start with, the energy sources we use, are entirely common through the cosmos.
00:52:20So, chemical reactions something like these must be responsible for the organic matter in interstellar space and the amino acids in the meteorites.
00:52:30Similar chemical reactions must have occurred on a billion other worlds in the Milky Way galaxy.
00:52:37Look how easy it is to make great globs of this stuff.
00:52:41The molecules of life fill the cosmos.
00:52:44Now, what would life elsewhere look like?
00:52:50Even if it had an identical molecular chemistry to life on Earth, which I very much doubt,
00:52:56it could not be similar, very similar, in form to familiar organisms on the Earth.
00:53:03The random character of the evolutionary process must create elsewhere creatures very different from any that we know.
00:53:11Think of a world something like Jupiter, with an atmosphere rich in hydrogen, helium, methane, water, and ammonia,
00:53:20in which organic molecules might be falling from the skies like manna from heaven,
00:53:25like the products of the Miller-Urey experiment.
00:53:28Could there be life on such a world?
00:53:30Well, there's a special problem.
00:53:33The atmosphere is turbulent, and down deep, before we ever come to a surface, it's very hot.
00:53:39If you're not careful, you'll be carried down and fried.
00:53:43So one way to make a living is to reproduce before you're fried.
00:53:46Turbulence will carry some of your offspring to the higher and cooler layers.
00:53:51Such organisms could be very little. We call them sinkers.
00:53:55The physicist E.E. Saul Peter and I, at Cornell, have calculated something about the other kinds of life that might exist on such a world.
00:54:08Vast, living balloons could stay buoyant by pumping heavy gases from their interiors, or by keeping their insides warm.
00:54:17They might eat the organic molecules in the air, or make their own with sunlight.
00:54:22We call these creatures floaters.
00:54:27We imagine floaters kilometers across, enormously larger than the greatest whale that ever was.
00:54:33Beings the size of cities.
00:54:36We conceive of them arrayed in great lazy herds as far as the eye can see, concentrated in the updrafts, in the enormous sea of clouds.
00:54:48But there can be other creatures in this alien environment. Hunters.
00:54:54Hunters are fast and maneuverable. They eat the floaters, both for their organic molecules and for their store of pure hydrogen.
00:55:02But there can't be many hunters, because if they destroy all the floaters, they themselves will perish.
00:55:12Physics and chemistry permit such life forms. Art presents them with a certain reality.
00:55:18But nature is not obliged to follow our speculations.
00:55:22However, if there are billions of inhabited worlds in the Milky Way galaxy, then I think it's likely that there are a few places which might have hunters and floaters and sinkers.
00:55:38Biology is more like history than it is like physics.
00:55:42You have to know the past to understand the present.
00:55:45There's no predictive theory of biology, just as there's no predictive theory of history.
00:55:49In history, the reason is the same. Both subjects are still too complicated for us.
00:55:54But we can understand ourselves much better by understanding other cases.
00:55:59The study of a single instance of extraterrestrial life, no matter how humble a microbe would be just fine, will de-provincialize biology.
00:56:11It will show us what else is possible.
00:56:14We've heard so far the voice of life on only a single world.
00:56:21But for the first time, as we shall see, we've begun a serious scientific search for the cosmic fugue.
00:56:29In recent years, we've learned much more about the origin of life.
00:56:45Do you remember RNA, that nucleic acid that our cells use as messengers carrying the genetic information out of the cell nucleus?
00:56:54Well, it's been found that RNA, like protein, can control chemical reactions, as well as reproduce itself, which proteins can't do.
00:57:05Many scientists are now wondering if the first life on Earth was an RNA molecule.
00:57:11And it now seems feasible that key molecular building blocks for the origin of life fell out of the skies four billion years ago.
00:57:20Comets have now been found to have a lot of organic molecules in them.
00:57:24And they fell in huge numbers on the primitive Earth.
00:57:29We also mentioned the extinction of the dinosaurs and most of the other species of life on Earth about 65 million years ago.
00:57:37We now know that a large comet hit the Earth at just that time.
00:57:41The dust pall from that collision must have cooled and darkened the Earth,
00:57:46perhaps killing all the dinosaurs but sparing the small, furry mammals who were our ancestors.
00:57:54Other cometary mass extinctions in other epochs seem likely.
00:57:58If true, this would mean that comets have been the bringers both of life and death.
00:58:05調� The Esta
00:58:06And Infection
00:58:07O
00:58:2031
00:58:2113
00:58:2215
00:58:2315
00:58:2415
00:58:2515
00:58:2616
00:58:2716
00:58:2918
00:58:3015
00:58:3217
Recommended
1:00:27
|
Up next
1:01:10
52:11
4:26
5:00
42:02
41:46
1:00:53
58:40
1:03:11
59:42
1:01:40
1:01:14
Be the first to comment