If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:10:26
SYI‑3 (EU)
SYI‑3.E (LO)

Video transcript

we finished off the last video in the Hadean Eon it was named for it was named for Hades or the ancient Greek underworld Hades is also the name of the God that ran the Greek underworld Zeus is Zeus's oldest brother and it was an appropriate name although the idea of that the ancient Greek notion of the underworld isn't exactly the the more modern notion of Hell but it was a hellish environment you had all this lava flowing around you had things impacting the earth from space and as far as we can tell right now it was completely inhospitable to life and to make matters worse even though the earth started to cool down a little bit maybe the solid the crust became a little bit more solid maybe the collisions started to happen less and less as we started to go a few hundred million years fast forward after Thea rammed into the the early Earth and formed the moon there was something called the late heavy bombardment and right now the consensus is that life whatever we are descended from would have had to come about after the late heavy bombardment because this was a time where so many things from outer space were hitting earth it was so violent that it might have killed off any kind of primitive self-replicating organisms or molecules that might have existed before it and I won't go into the physics of the late heavy bombardment but we believe that it happened because Uranus and Neptune so this is the Sun right here that is the Sun this is the asteroid belt this is the asteroid belt that's outside that's outside the orbits of the inner rocky planets that Uranus and Neptune their orbits their orbits moved outward their orbits moved outward and I'm not gonna go into the physics but what that caused is it caused some it gravitationally it caused a lot of the asteroids in the asteroid belt to move inward and start impacting the inner planets and of course earth was one of the inner planets and I should make the Sun I should make the Sun like orange or something not blue I don't want you think that's earth and also impacted the moon and it's more obvious on the moon because the moon did not have an atmosphere to kind of smooth over the impact so the consensus is that only after the late heavy bombardment was earth kind of ready for life and we believe that the first life formed 3.8 to four billion years ago remember G for Giga four billion years ago when we talk about life at this period we're not talking about squirrels or panda bears we're talking we're talking about extremely simple life forms we're talking about prokaryotes and let me give you a little primer on that right now that we go into much more detail in the biology playlist pro4 oh we're talking about prokaryotes and I'll I'll compare them to eukaryotes you carry oats you carry oats prokaryotes are for the most part unicellular unicellular organisms that have no nucleuses they also don't have any other membrane-bound what we'd called organelles or these little parts of the cells that perform specific functions like mitochondria so their DNA is just kind of floating around let me draw this character's DNA so it's just floating around just like that and prokaryote before kernel or before a nucleus eukaryotes eukaryotes do have a nucleus where all of their DNA so this is this is the nuclear membrane and then all of its DNA is floating is floating inside of the nucleus and then it also has other membrane-bound organelles mitochondria is kind of the most famous of them so it also has things like mitochondria we'll learn more about that in future videos mitochondria we believe it's essentially one prokaryote crawling inside of another but prokaryote and becoming and kind of starting to become a symbiotic organism with each other but I won't go into that right now but when we talk about life at this period we're talking about prokaryotes and we still have prokaryotes on the on the planet bacteria and archaea are examples of prokaryotes and just to give you a little bit of a tidbit right here this is kind of a this kind of shows our current understanding of where we think things branched off from so at this point of the tree is some common ancestor to prokaryotes and eukaryotes so these are the prokaryotes right over here the bacteria in the archaea and here is the eukaryotes and this first living thing or this first set of living things we think might have just been some type of self-replicating molecules and slowly some membrane might have come around and it became a little bit more organized DNA RNA maybe that maybe RNA was that original self-replicating molecule became the Metin the method of kind of transmitting information from one generation to the next so it's really still an open question of exactly what what that first life is or even how do you define that first life but based on based on studying the genetic makeup of molecules of the current organisms this is how we think the Tree of Life came about so we have one common ancestor then they broke apart and then the archaea and eukaryotes have a common ancestor that's different from the bacteria and we'll talk more about that in the future and this right here just so you can visualize it this is an example of bacteria this is this is e coli or ISA ricci coli that's just an example of bacteria comes in a bunch of shapes and forms but it's a pro carry otic life form and the earliest life forms the earliest life forms we also think we're anaerobes these are things that did not need one that they did not need oxygen and they for the most part found oxygen poisonous and the earliest life forms also probably did not perform photosynthesis they might have gotten their energy from other sources chemically from this kind of extremely volatile environment that they were in at that time so if we fast forward a little bit and this is actually a major event in the history of Earth and these are huge timescales we're talking about I mean remember I'm kind of just nonchalantly saying Oh 4.6 billion years ago - 3.8 billion you know that's just 800 million years remember and I'll talk about this you know you know a grass has only existed for 50 million years this is 800 million years humans and champion chimpanzees only diverged 5 million years ago this is 800 million years we're talking about you know from from from ancient Greece to now we're only talking about about 2500 years 2500 years you multiply that times a thousand you multiply that times a thousand you get two and a half million years so we can and and this is 800 million years we're talking about so these are extremely huge periods of time and that's why we call them eons you know eons are 500 million to billion years now the dividing line between the Hadean Eon and the Archaean Eon and it's kind of a fuzzy dividing line but most people place it about 3.8 billion years ago is kind of before the art is kind of the earliest rocks that we can observe and so we can we have rocks from that are roughly 3.8 billion years ago so we kind of put that as the beginning of the Archaean Eon and so there's two things there one rocks have survived from the beginning of the Archaean Eon and also that's roughly when we think that that the first life existed in there so we're now in the Archaean Eon and you might say oh maybe Earth is a more pleasant place now but it would not be it still has no too little oxygen in the environment if you were to go to earth at that time it might have looked something like this it would have been a reddish sky you would have had you would have had nitrogen and methane and carbon dioxide and the atmosphere there would have been nothing for you to breathe there still would have been a lot of volcanic activity this right here these are pictures of stromatolites stromatolites stromatolites and these are formed from these are formed from bacteria that are bringing in sediment particles and over time these things get built up but the most significant event in the Archaean period at least in my in my humble opinion was what we believe started to happen about 3.5 billion years ago and this is prokaryotes or especially bacteria evolving to actually utilize energy from the Sun to actually do photosynthesis and the real the real fascinating byproduct of that other than the fact that they can now use energy directly from the Sun is that it started to produce oxygen so starts start to produce produce oxygen and at first this oxygen even though it was being produced by the cyanobacteria by this Bluegreen bacteria it really didn't accumulate in the atmosphere because you had all of this iron that was dissolved in the oceans and let me be clear all of the life that we're going to be talking about for really the next several billion years it all occurred in the ocean we had no ozone layer now you the land was being irradiated the land was just a completely inhospitable environment for life so all of this was occurring in the ocean and so the first oxygen that actually got produced it actually was it actually got it actually instead of just being released into the atmosphere it ended up bonding with the iron that was dissolved in the ocean at that time so it actually didn't have a chance to accumulate in the atmosphere and we're going to when we fast-forward past the Archaean period we're going to see that once a lot of that iron was oxidized and the oxygen really did start to get released in the atmosphere it actually had a you know it's funny to say a cataclysmic effect or a catastrophic effect on the other life of the other anaerobic life on the planet at a time and it's funny to say that because it was a catastrophe for them but it was kind of a necessary thing that had to happen for us to happen so you know for us it was a blessing that this cyanobacteria started to pump out a lot of oxygen and eventually oxidized all of the iron and eventually released a lot of oxygen in the atmosphere and killed off all of this anaerobic bacteria so that eventually we could you know us us us oxygen breathing organisms could come about but that's not going to happen for a while we still have a few billion years before things start things start flopping around on the land anyway see in the next video
Biology is brought to you with support from the Amgen Foundation
AP® is a registered trademark of the College Board, which has not reviewed this resource.