Placeholder Image

字幕列表 影片播放

  • Hi, I'm John Green,

  • and welcome to Crash Course Big History

  • where today, we're going to get a life.

  • Or at least, the earth is going to get a life.

  • But first, today we have to start with a disclaimer.

  • The origin of life is in many ways a blank spot

  • in the pages of history.

  • Like the mystery surround the Big Bang or dark matter,

  • the origin of life is still pretty puzzling to us.

  • Like, thanks to scientific research,

  • we have a general idea of what needed to happen

  • to bring about life, but we're pretty fuzzy on the details.

  • Mr. Green, Mr. Green!

  • I mean, if we don't know,

  • then why are we studying it as history?

  • Maybe we should just, like, let scientists

  • figure all that stuff out and then they'll get back to us,

  • like, after this class is over.

  • Well, me from the past, I'm sure the thousands of scientists

  • working on that question appreciate your patience,

  • but even when we have blank pages in the annals of history,

  • it's still history.

  • Like, there are still competing ideas and theories

  • about the presidency of Franklin Delano Roosevelt,

  • but the fact that there are open questions doesn't mean

  • it didn't happen.

  • Sometimes we don't have a clear narrative of events

  • and it's up to us to collect more evidence

  • and refine those theories, but first,

  • we have to know about the current evidence

  • and the current theories.

  • I mean, ultimately, that's what history is.

  • >> I'm Hank Green.

  • And this is still Crash Course Big History.

  • Last time, we left off with a newly born Earth

  • that was molten hot, pelted by asteroids.

  • Then, millions of years of torrential rainfall

  • cooled the surface and created the first oceans.

  • We know that life emerged in the oceans

  • between 3.5 billion and four billion years ago.

  • We have solid fossil evidence for life 3.5 billion years ago

  • and many scientists are pretty confident

  • that life was around 3.8 billion years ago.

  • It's pretty clear that life is a different thing

  • from the rest of the universe,

  • but what makes up that difference?

  • I'm kind of surprised that this turns out

  • to be a super puzzling question that we have yet

  • to come up with a 100% satisfying answer to.

  • But some of the major characteristics

  • of most life are, it adapts to the environment,

  • it has a metabolism that processes energy

  • to keep itself going-- like humans do with pizza--

  • and it reproduces, whether it be a cell splitting in two

  • or two animals... doing their thing in nature.

  • Even these simple criteria have their problems, though.

  • Some animals, like mules are born unable to have offspring.

  • Some microorganisms can shut down their metabolisms

  • for long stretches of time, but neither are exactly dead

  • or not life.

  • Given the incredible variety of species,

  • definitions for life are, by necessity, very broad,

  • but one such definition by big historian Fred Spier is,

  • and I quote, "A regime that contains a hereditary program

  • "for defining and directing molecular mechanisms

  • "that actively extract matter and energy from the environment

  • "with the aid of which matter and energy are converted

  • "into building blocks for its own maintenance and,

  • if possible, reproduction."

  • (sighs)

  • In other words, what makes you different from the stars that,

  • while a stars burns down till it dies and doesn't actively float

  • around the cosmos looking for more fuel,

  • a living organism does actively seek out pizza

  • to keep itself going, preferably long enough to,

  • you know, have some babies.

  • But how do we know what we know?

  • How do we know that life is just a different kind

  • of molecular mechanism and not something more profound?

  • Well, we can test these claims, and we do, using science.

  • Because life looks so radically different

  • from the inanimate universe, people once thought

  • that life was made of completely different stuff.

  • Then, in 1828, a German chemist, Friedrich Wohler,

  • used inorganic chemicals to synthesize an organic chemical.

  • This was a big deal just as Newton's theory of gravity

  • showed that the heavens and earth followed

  • the same physical laws, Wohler's experiment proved

  • that life and non-life follow the same chemical laws,

  • which implied that life could emerge from non-life.

  • Even this idea wasn't completely new.

  • For centuries, the Aristotelian idea that life

  • just spontaneously emerged from non-life was widely believed.

  • For example, if you put some rotten meat out in the sun,

  • eventually the meat would transform itself into maggots.

  • You could probably work out the weaknesses in this theory.

  • 17th century scientists took meat and various other objects

  • thought to spontaneously generate life,

  • boiled them to kill off any eggs previously laid by insects,

  • sealed them in jars and nothing happened.

  • Oh Aristotle, first you told us that snot was our brain

  • coming out of our noses, and now you made all those

  • nice people waste their steak dinner.

  • This however did not rule out some form of life force

  • in the air.

  • Some invisible force in the earth's atmosphere

  • that could enter an object and literally breathe life into it.

  • But spores from plants can also travel in the air,

  • as can microorganisms.

  • So in the mid-19th century, Louis Pasteur boiled

  • some organic broth friendly to life and placed it

  • in a flask with a swan neck to trap plant spores

  • and smaller particles.

  • If a life force was in the air, it could enter freely

  • while spores and other particles would get trapped in the U-bend.

  • And what happened?

  • Nothing.

  • A century and a half later

  • those flasks are still devoid of life.

  • The conclusion: The ancients were wrong.

  • After a dose of claim testing, it became clear that life

  • must emerge from the inanimate world by chemical processes

  • that are discoverable by science.

  • But what did early life look like?

  • Well, for a whopping 2.1 billion of the 3.8 billion years

  • of the evolutionary epic

  • history was made by tiny, single-cell organisms

  • called prokaryotes.

  • That's roughly 55% of the entire story of life.

  • Now some of those prokaryotes evolved about 1.7 billion

  • years ago into slightly bigger single-celled organisms

  • called eukaryotes.

  • And then, you know, that kept happening and eventually us.

  • But for now, let's just talk about prokaryotes.

  • Prokaryotes lived in the seas and ate chemicals

  • in their surrounding environment.

  • Now these microscopic prokaryotes might not sound

  • very impressive, but they do make up

  • the vast majority of your family tree.

  • They're also distant relatives of the modern bacteria

  • that are everywhere.

  • Crawling around the room that you're in right now,

  • crawling all over you, crawling inside of your intestines.

  • That's right, somewhere right now there's a bacterium

  • that will give you food poisoning

  • in an undercooked hamburger, and it is your cousin.

  • But the thing is, even in its earliest stages,

  • single cell life was massively complex compared

  • to the inanimate universe.

  • I mean I know these are tiny little specks,

  • but compared to everything else that had happened on earth

  • until them they were an immense tangle of chemical networks

  • and building blocks.

  • But how did an object as ridiculously complex

  • as a prokaryote first emerge?

  • Well, first of all, it's very difficult to think

  • of how life would form in an oxygen-rich atmosphere

  • like present-day Earth's.

  • Oxygen is kind of a nasty, highly reactive chemical.

  • In fact, if the oxygen levels in this room were

  • substantially higher and I would just rub my hands together

  • really fast, I could burst into flames.

  • And while that would make for a nice viral YouTube video,

  • I would rather not be on fire than get lots of views.

  • 3.8 billion years ago, the free oxygen content of the atmosphere

  • was at negligible levels,

  • which had some not so pleasant consequences.

  • For millions upon millions upon millions of years,

  • life dwelled fairly deeply in the ocean,

  • eating chemicals and staying where

  • the earth's heat kept warm.

  • Eventually, some prokaryotes floated near to the top

  • of the ocean and started using sunlight, water,

  • and the carbon dioxide that was abundant

  • in the earth's atmosphere to sustain their own complexity

  • using this sweet chemical process they'd come up with

  • called photosynthesis.

  • The waste product of this chemical process is oxygen.

  • And these photosynthesizing prokaryotes pumped a lot of it

  • into the atmosphere.

  • By around 2.5 billions year ago, the amount of free oxygen

  • in the atmosphere was up to about 3%.

  • Oxygen can be nasty and so scores and scores

  • of tiny single-celled organisms couldn't handle it

  • and died off in a massive wave sometimes known

  • as the oxygen holocaust.

  • So many species of single-celled organisms,

  • each with the potential to evolve into more complex life

  • were wiped out.

  • Even at this early stage, our evolutionary ancestors

  • were squeezed through a bottleneck.

  • And this will not be the last such disaster

  • that nearly wiped everything out.

  • Next time you have a bad day remember that it is amazing

  • that you are alive at all, much less a member

  • of a self-aware species living at the height

  • of human technological progress.

  • Speaking of ancestors, somewhere between 1.6 and two billion

  • years ago, the eukaryotes evolved.

  • And because you, your dog and the chicken you ate last week

  • and the mushroom you ate the week before all descended

  • from them, they really put the "you" in eukaryotes.

  • And eukaryotes contained organelles like cellular organs

  • that enhanced their abilities.

  • About 1.5 billion years ago, eukaryotes invented sex.

  • Up until that point, single-celled organisms

  • split in two or cloned with no need to find a partner

  • for romance and DNA exchange.

  • Sexually reproducing eukaryotes possibly obtained

  • these abilities through cannibalism,

  • just eating each other,

  • which may have led to some accidental exchange of DNA.

  • After that, the evolutionary advantages of sex

  • probably resulted in it catching on.

  • Having a partner means having two sets of genes

  • and thus a wider range of genetic diversity

  • from which evolution can pick and choose.

  • Sex is a huge deal.

  • It enhanced evolution, and therefore deserves to be classed

  • as one of the most revolutionary advances in the history

  • of life on earth.

  • And a huge leap forward in the rise of complexities

  • since the very beginning of the universe.

  • So where did these complex single-celled organisms

  • come from in the first place?

  • Well, Charles Darwin's own hypothesis was that life

  • evolved in some "warm little pond suitable

  • for fostering life."

  • Other scientists postulate that life may have formed

  • from organic chemicals next to the warmth

  • of underwater volcanoes.

  • And still others champion the idea of panspermia,

  • which states that life may have evolved elsewhere

  • in the solar system and then been transported here

  • by an asteroid, which seeded the earth.

  • Like I said, this is a blank spot where many different

  • historical theories are seeking evidence

  • to clarify what happened.

  • It's possible actually that this problem could be solved

  • in our lifetime, which is pretty exciting.

  • Anyway, whatever physical forces were at play,

  • primitive organic chemicals eventually came together

  • into balls with protective membranes.

  • They would have reproduced and proliferated

  • much as life does today, but the earliest blobs

  • or organic chemicals would have reproduced clumsily,

  • inaccurately with many useful adaptations getting lost.

  • Essentially, these molecular mechanisms

  • were badly programmed.

  • In 1950s, James Watson, Francis Crick, Maurice Wilkins,

  • and Rosalind Franklin discovered how living cells replicate

  • using DNA or deoxyribonucleic acid.

  • DNA is a double-stranded molecule that contains

  • a list of orders for how it wants a living cell