We wouldn't be human beings
is we didn't wonder how everything began. There are, of
course, many religious explanations for where it all came from;
science tends to shy away from these not because scientists are
non-religious, but because those supernatural explanations can
really be tested in any way, while scientific explanations can be.
Scientists also aren't that interested in a concept called
panspermia: this is the idea that the earliest life began
someplace other than Earth and was somehow carried here. Also
not very testable (although some researchers believe that viable
bacteria have been found sealed in meteors). We can't travel
backward in time, but we can make predictions based upon hypotheses
and test the predictions.
first began to think about how Life on Earth began, they looked at
how Life worked around them, and everything depended upon fuel from
plants to exist. The process of photosynthesis is very
complex, and its appearance as a first step would have required some
sort of supernatural intervention: this was known as the
photosynthesis or plant problem.
It took better knowledge of how the Earth formed,
and the chemistry of the early Earth, to solve that problem.
The space dust that would have been coalesced to form the Earth, it
turns out, has a decent proportion of organic molecules already in
it. What emerged from that discovery was the Heterotroph
Hypothesis, based on the idea of a Primordial Soup, a
planet-covering solution of simple organic molecules, exposed to
many types of energy, from lava heat to ultra-violet light to
lightning in an extremely active atmosphere (it's still pretty
active today, with an estimated 100 bolts per second planet-wide).
The question was, could small organics, given such conditions, form
larger organic molecules, and larger yet, until they became
cooperative systems, able to self-organize, reproduce, and evolve?
This is still an open question, but many small predictions based
upon the hypothesis have been confirmed.
Some discoveries: RNA was found to have
enzyme properties, providing a first possible living-system molecule
that could have led to protein chemistry and DNA coding; some
components of stardust were found to be lipids, critical in the
formation of cell-like membranes; hydrothermal vents, with a
fairly stable supply of raw materials and energy, and ecosystems
based upon relatively simple chemosynthesis, were found; a
primitive form of photosynthesis was found in bacteria near
hydrothermal vents; a layer of rust in the fossil record
indicated a rough date for the appearance of widespread release of
oxygen from photosynthesis, after indications that life already had
existed for some time; chemistry very much like the
hypothesized primordial soup can be found elsewhere in the solar
system, such as the moons of Jupiter and Saturn.
Unless we find a world in the early stages of
developing its own living systems (which might be happening deep in
those moons), we can't be too sure that it works the way the
hypothesis says, but the evidence slowly mounts.
A broad assortment of creation stories.
Some more on Primordial Soup.
Description of one of the first experiments about the first
How'd it get to be RNA?
More on that primitive photosynthesis.
And could conditions found on other planets or moons produce a type of Life?
The basic processes of life
were first developed in prokaryotes - they were the first
photosynthesizers, the first aerobes. They spread across the
globe, but they were limited. Prokaryotes can be found in
simple colonies, and different species can work cooperatively
together, but they can't do the complex jobs that eukaryotes can.
Particularly, they can't form multicellular systems.
It's unclear when
eukaryotes, with their specialized chambers, evolved, or when they
added to those chambers by taking in specialized prokaryotes and
using them (the endosymbiont theory), but most evidence
indicates that even eukaryote life stayed pretty simple for a very
long time. Plants formed simple algae sheets and mats;
animals stayed small and soft; neither left much in the way of
fossils. The Earth's oceans were apparently a stable ecosystem
for a very long time. Life might have stabilized in those
Then it all froze.
The Snowball Period
was the worst ice age ever - even the tropical oceans were covered
thickly with ice. No one knows for sure why it started, but it
had to have left few decent places to live, reduced the producer
population drastically, and set off a wave of competition not seen
for eons. It lasted for millions of years.
accumulating with few plants to remove them, have a greenhouse
effect, and that's what probably caused the eventual thaw.
From whatever isolated refuges they had used, the surviving animals
- now changed - spread out. In the fossil record (the few
surviving examples), many new types of animals seem to appear almost
instantly; this is called the Cambrian Explosion.
All of the major modern animal phyla (and some that wouldn't survive
to today) are found in these fossils.
Life was still confined
to the oceans, though, so plant life did not have an explosion (and
it's hard to tell, since they fossilize poorly). For hundreds
of millions of years, life in the ocean developed, some groups
moving into the watery ecosystems of the continents like tidal pools
and fresh water systems. There, abilities arose that could
also be used out of the water: resistance to drying / osmosis;
resistance to direct sunlight and rapid temperature changes;
exposure to the oxygen levels that had accumulated in the atmosphere
(water can hold much less oxygen); moving from pool to pool or
against strong currents with sturdier structure.
From these two systems came the movement of
Life onto the land of the continents. This is when plants
made their major evolutionary leap forward, but many animal groups
went with them.
occasional catastrophic events that killed most species on the
planet, often cleared the systems, and what moved back in often came
from different groups, such as when the mammals and birds replaced
the massive reptiles after the Cretaceous Extinction. In each
extinction, something happened to wipe out most of the plants:
shading dust from asteroid impacts or major volcanic eruptions, or
ice ages. And the food pyramid can't stand with its base
Regional extinctions sometimes occurred
when separate ecosystems connected (as when Panama formed between
the American continents), or major climates changed (that connection
dried out much of Africa, providing new niches that our own
ancestors exploited, evolving from small forest dwellers to walking,
hunting creatures of the grasslands).
unlikely to bring about major extinctions is disease. Diseases
may wipe out small vulnerable populations of particular species, but
two things restrict their extinction potential: first,
diseases virtually never affect wide ranges of different types of
organisms (and even with small ranges, the effects vary from deadly
to nothing); second, diseases themselves evolve. They
need to spread offspring, and the less ill they make their hosts,
the more likely that is to happen. In most cases, diseases are
most serious when they move into a new host type (they aren't
well-adapted to the new system and mess up its chemistry), but over
time the disease will get more and more mild. It may produce
illness symptoms to help its offspring spread (like a cold or flu
making us cough and sneeze), but these are rarely serious.
from an earlier chapter.
More on the endosymbiont theory and early developments in Life.
Early algae evolution.
Evidence for earliest animals.
Evidence for Snowball Period (but was it really snowy?).
First in a series on early Earth conditions, including Snowball.
Animals from the Cambrian Explosion.
Cambrian Explosion explained.
Plants move to land.
Major extinction events.
Extinction events and living groups.
Can humans bring about a mass extinction?