Hi, everybody. Welcome back.
Today we're covering the content
that is in Chapter 25 of your textbook,
and this chapter largely covers some broad patterns
with regard to the history of life on the planet.
When we were together last time, we were looking at mechanisms
that could promote speciation, right?
We talked about allopatric speciation
and sympatric speciation.
Today, we're going to look at,
again, some broad-scale events that occurred
over the history of life on the planet
that include things like mass extinctions,
and the phenomenon of what we call adaptive radiation
where we see many, many species
show up in-- in the fossil record.
And yeah, a lot of what we know with regard to what
we're going to talk about today comes from an exploration of
and interpretation of that fossil record.
Starting with this slide right here:
this is a amazing photo of a skeleton of a whale
that was discovered in the Sahara Desert.
So, you might be wondering, well, how did that happen?
Researchers were-- were knowing where to look
when they were-- when they were trying to figure out
what are the origins of marine mammals, for example.
And so, due to conversations with geologists about,
you know, where we might find fossils
of-- of particular creatures from a particular time
in the history of life on Earth,
and other patterns of-- of events that--
that led researchers to predict
that they might actually find whale bones
in the-- in the desert of the Sahara.
And, in fact, they did.
So, that's just one example of some of the work
that paleontologists do.
Paleontology is the study of the fossil record.
So, yeah, let's go ahead and get started
building on the work that-- that we have learned
from paleontologists today.
So, again, when we use the word macroevolution
now we're really talking about broad patterns of evolution
that-- that are above the species level.
So, we're looking at groups of organisms.
And when we look in the fossil record, we do see some trends.
You know, we see the emergence, for example,
of terrestrial vertebrates.
We see the emergence of other groups of species as well,
that I'm going to share with you today.
We also see that mass extinctions occurred.
And then there's been, you know,
there's been...
you know, that has affected
the evolutionary trajectory of other species.
So, we'll talk about that today.
And then we'll focus on some key adaptations
such as flight, for example.
So, when these new adaptations arise,
it allows for what we call "adaptive radiation".
I'll talk about that today as well.
The possibility for many new species
to emerge over a relatively short period of time,
and I'm talking geologic time.
So, you know, short is relative here in this conversation today.
But, yeah, when you have a novel character that arises,
such as the ability to fly,
that promotes the possibility
for the--
for a species that has that particular character
to diverge into many other species
as they occupy new habitats that are available to them
because of that character that emerged.
So, yeah, we'll talk about each of these in detail today.
All right, if we're going to have a conversation
about the history of life on Earth,
we might as well start at the very beginning.
So-- So, one of our big questions is
what is the origin of life?
How did life ever get started on planet Earth, right?
And so, I'm going to share with you some evidence
that supports the idea
that the origin of life
occurred via these sequential steps
that you see in my slide here.
So, it makes the most sense to us,
and the data support this idea,
that the first thing that probably happened
was number one here, abiotic synthesis.
Abiotic meaning non-living, right?
Abiotic synthesis of very small organic molecules,
probably monomers of molecules.
And then, finally, polymers.
The joining of those together
to make those polymers probably occurred.
And then, at some point, those--
those molecules were probably captured inside
what we colloquially call, "protocells," right.
Proto- meaning first, these-- these precursors to modern cells.
And then lastly,
there is evidence to suggest the origin
of self-replicating molecules,
similar to what we see today, right?
Self-replicating DNA and RNA.
All right, let's take a look at each one of these
in detail today.
So, one of the first things that would have to happen,
according to that sequence of events
that I shared with you previously
is the abiotic synthesis
of organic molecules.
And researchers in the 1950s were curious
if that-- if they could get that to occur.
and, you know, like, the conversations at the time were
well, what did planet Earth's atmosphere
look like at the time?
Let me back up a minute.
We think that the planet formed about 4.6 billion years ago,
but for that first half a billion years,
so from 4.6 billion years ago to 4 billion years ago
the planet was probably not conducive
to life ever forming on the planet.
There was-- it was constantly being bombarded
by rocks and debris, and it was a very hot environment.
But about 4 point-- billion years ago,
4 billion years ago,
the planet cooled off, the seas formed,
and the environment was--
the atmosphere, we don't know exactly what it looked like,
but we have some indication
that there was methane in the atmosphere,
and there was ammonia in the atmosphere,
and there's hydrogen gas in the atmosphere.
And, whether or not it was a-- a reducing environment
or an oxidizing environment.
Not sure.
That refers to whether--
you know, today we're living in an oxidizing atmosphere, right?
There's oxygen gas in our atmosphere that will readily oxidize
other compounds, right--
steal electrons away from-- from other compounds.
There was some indication recently
that the environment was, in contrast to that,
a reducing environment full-- full of hydrogen gas,
like you see here, H2,
that would donate electrons to compounds.
So, regardless of that,
this experiment right here shows you an apparatus
that was set up by a grad student
from the University of Chicago in 1953.
His name was Stanley Miller.
He and his-- his advisor worked on this project
where they-- they attempted to simulate
what they thought was the atmospheric and oceanic conditions
on the planet 4 billion years ago,
to see if they could get the abiotic synthesis
of organic molecules.
So, what you see in this apparatus
is a container
that might simulate the ocean.
And, you know, we know from volcanic activity in the ocean
that there are areas where it's-- it's very hot.
So, they-- they simulated that,
you know, providing thermal energy.
And then, you know, some of that water
would, of course, evaporate.
And this part of the chamber, here,
is kind of representing
what we think were atmospheric conditions.
Here's methane, here's ammonia, here's hydrogen gas.
He simulated lightning
with these electrodes here,
again, providing energy.
And then, as that water vapor cooled--
here's a condenser here.
input cold-- cold water to cool it off
to simulate a cooler atmosphere.
And then as that water condensed,
he sampled it from time to time.
And, honestly, his graduate advisor
actually thought that there was no way that they
were going to get any kind of organic monomers anytime soon.
You know, we thought that, you know,
if this happened on the planet 4 billion years ago,
how many million years did it take
to get synthesis of these organic molecules?
But, lo and behold, they actually got results
in a matter of months.
Three or four months later,
they found in their sample,
when they sampled it for chemical analysis,
they found amino acids, amazingly enough.
So, this did represent,
you know, the phenomenon that abiotic synthesis
of organic molecules can indeed occur.
This was an exciting moment
because it set the stage for--
you know, it set the stage chemically
for the conditions for life.
Another burning question, so to speak,
that we have is, well, where did life originate on the planet?
A lot of interested-- a lot of interest is being paid
to these alkaline vents that are in the deep sea.
Here's a picture of one of these vents in the slide here.
These vents release water with a very high pH;
9, 10, 11.
So, they're considered alkaline vents.
And also very warm water, 40 to 90 degrees C.
And the conditions in these vents
were probably very likely suitable
for the formation of some of these organic compounds.
And, indeed, researchers have looked at the surface
of the structures that formed these vents
and found organic molecules attached to those vents.
So, maybe this is where life arose.
Other people are very interested in looking at meteorites.
Meteorites may have been another source of organic molecules.
For example, the fragments of a meteorite
called the Murchison meteorite,
has been discovered to contain more than 80 amino acids
and other key organic molecules,
including lipids, some sugars, also some nitrogenous bases.
So, maybe these are the-- maybe we can--
maybe we can consider that that is
where we got these first organic molecules
is from the meteorites that have bombarded the planet.