One theory, panspermia or the space seed hypothesis, proposes that life can be found on meteors and other space debris, and such objects brought the first Life to Earth. In fact, some very rigorous tests suggest that there may be bacteria in space, perhaps left from some long-destroyed planet and perhaps capable of surviving a leisurely trip across the universe. If this is the case, then no one could be sure of the conditions under which that life would have first evolved. This is a possibility, but a bit of a dead end explanations-wise.
Another theory is the special creation theory, that supernatural forces (pick a candidate from a long list) put the first prokaryotes together as part of the long building plans which would ultimately lead to us. Science tends to be resistant to supernatural explanations of things, but when youre dealing with conditions so far in the past, there is not much less evidence for some aspects of this idea than many of the others.
What we are going to look at are the theories
that assume that Earths life is "home-grown," emerging
from those earliest conditions to exist eventually as what we see
When biologists first began to wrestle with these concepts, the ideas of uniformitarianism were still powerful, and people theorized in terms of a world not all that different from the world they knew. Our worlds ecosystems depend almost exclusively upon photosynthesis to construct the fuel that all life runs on; in an ancient world with conditions similar to todays, it seemed that you would need plants (as organisms that can make complex molecules using simple building blocks and energy available from the environment, plants are known as one type of autotrophs, or "self-feeders") to evolve first, or there would be no bottom link to the food chain. This was an insurmountable problem to theorists, because the processes of photosynthesis are much too complicated to have spontaneous formed from "nothing" under present-day conditions (or any conditions).
But through the first half of the 20th Century,
studies from astronomy and geology suggested that the very early
Earth was a dramatically different place than it was today.
It was a hotter and nastier place, heated by a warmer sun and a
warmer interior. This suggests more violent weather, with
huge storms and lots of lightning. The atmosphere had no or
almost no oxygen, so no ozone layer to absorb the suns
ultraviolet rays. Simple organic materials, which are
in the space dust and debris that the Earth formed from, would
have filled the oceans with a kind of brownish gunk, potential
building blocks of life. This is known as primordial
soup, and this concept of the early Earth led to something
called the heterotroph hypothesis (heterotrophs
require their complex fuel molecules already made, unlike
autotrophs). Some interplanetary missions are looking for similar
circumstances elsewhere in the
solar system, where life may also have arisen.
The ability to self-organize. This requires some already-formed building blocks, from the soup, and a source of energy that would serve to help drive them into increasingly complex forms. Experiments in the early 1950s began to confirm that such processes could at least begin. Those experiments used lightning, confined to a "primordial soup bottle," to stimulate production of complex materials. Since that time, all of the various forms of energy available on the early earth have been tested, against more well-understood formulas for the soup, with varying results. The current "leading contender" for life-organizer are hydrothermal vents, openings between Earths surface plates at the bottom of the oceans. There, water mixes with hot magma and releases a hot soup of materials even today. They have an energy source - heat - a source of materials - once soup, now magma - and, perhaps most importantly, are a stable, long-lasting ecosystem and a place to "work the bugs out" of the earliest living systems.
The ability to reproduce. As these early self-organizing molecules grew, only those which could make and spread copies of themselves had any real future. Life on todays Earth uses DNA code to store all of the information it needs to make the proteins it actually runs on, but DNA has little activity beyond that, and proteins generally cant duplicate themselves. There are theories that try to address those problems, but the leading current theory is that the first really complex systems were of RNA, a hypothesis usually called the RNA World hypothesis. RNA has DNAs coding abilities and some protein-like activity, and it isnt difficult to see the evolution of a DNA-coded protein system growing quickly from an RNA-based ancestor.
The ability to evolve. Once you
have a planetwide ocean full of self-organizing molecules able to
reproduce themselves, you have a competitive ecosystem where
selection can take place. This stage of
evolution would have favored those who could work most
efficiently, or best accumulate building blocks, or reproduce the
fastest, or work with other molecules in a cooperative fashion,
perhaps linking RNA or DNA codes for particularly good proteins
together to work as a unified system. A combination of RNAs like
those that work in cellular protein production (a little one holding amino
acids and a bigger one to put those amino acids in a particular order)
might have been especially useful and definitely on the track to
Life-as-we-know-it. And these unified
systems might work even better with some confinement and
But that didnt happen on Earth, because autotrophs, they who were eventually to become the whole basis of the food chain, evolved. Most likely, the first autotrophs were able to use the heat from hydrothermal vents in especially efficient synthetic ways, a process called chemosynthesis. Simple organisms performing this process can be found today in hydrothermal vents and some hot springs.
But how to get to photosynthesis, a system that also uses energy from the environment to construct fuel molecules, but one using light, a very different from of energy than heat? One possibility is that, in order to stay near their vent homes (which are like oases in a desert, if displaced by the turbulent currents it would be good to find your way back), some organisms developed ways to detect the faint glow the vents produce as a homing beacon. Once you have a chemosynthesis system and a light-reactive system in the same organism, its easier to imagine development of a system that could use light as an energy source for synthesis. This would be a huge advantage: chemosynthesis-based ecosystems, even in the early earth, would have been few and far between, but the entire surface of the oceans would have access to light.
Recent research has indeed found a type of bacterium that lives in hydrothermal vents and uses the tiny amounts of light there for photosynthesis.
So what became of the various molecular complexes of the earlier primordial soup? It is assumed that they went extinct, outcompeted by the more efficient cellular life now formed. But do we know this for sure? Its unclear whether anyone has actually searched for such entities in the oceans, which are filled with protein complexes, but such material is assumed to be a combination of proteins lost from living things, and viruses.
And what of
If some primordial molecular complexes survived by becoming parasites in
the cells that had evolved around them (there may have already been complexes that existed
by using other complexes abilities against them). They might have
eventually evolved (as parasites do) to have only hints of their ancestry,
to become so efficient in using the cells machinery (with its efficient
DNA-to-protein equipment), even to surrounding themselves with bits of
host membrane (many viruses have non-membrane coatings, like one might
expect from those primordial complexes) so as to seem more like degenerate
cells than a continuation of the very first living systems.
Respiration is a system by which fuel molecules from food are converted to the fuel molecules that power cells, mostly in the form of a molecule called ATP (Adenosine triphosphate). Various approaches would have existed long before the rise of photosynthesis, but the buildup of oxygen and wide availability of the simple sugar (glucose) produced by photosynthesis favored the rise of a particularly efficient respiration system that was almost a mirror image of photosynthesis: aerobic respiration.
So at this stage of Earths history, the oceans
would have been full of photosynthesizing prokaryotes, aerobic
prokaryotes, and predatory prokaryotes feeding off the
others. And a truly diversified world was about to get even
An ingested prokaryote can be digested and
absorbed, but sometimes they might be more useful
alive. A photosynthetic prokaryote could make food in the
sun for its captor, and an aerobic prokaryote could help it better utilize
the food it took in. The prokaryotes benefited, too:
beyond not being digested was a good thing, and becoming part of the larger predatory
cells certainly reduced their potential as prey. This type
of mutual-benefit relationship is called a symbiosis, and that absorbed prey might be used rather than digested is
reasonable - it can be
found in a few of todays
The vast majority of present-day eukaryote cells contain aerobic
respiration chambers called mitochondria that
structurally resemble aerobic bacteria, even down to having the
remnants of bacterial chromosomes in them, and eukaryote plant
cells contain photosynthesizing chloroplasts with
similar resemblances to a type of photosynthetic prokaryote.
That these structures began in the way described here is known as
theory, proposed in the late
1960s by Lynn
Margulis, then at Boston University, and eventually
widely accepted. Other eukaryote structures might have
endosymbiont origins, but the evidence for those is more
Prokaryotes can and often do live in groupings - a type of mineralized bacterial surf structure, stromatolites, shows up in very ancient fossils and can be found living today - but they rarely specialize. Its more like they "hang out" together. Eukaryotes, perhaps because with nuclei they are better able to control what genes get expressed in a particular cell, seem to have a talent for specialization.
For a very long time, the only multicellular forms in the fossil record were algae, barely above the colonial stage. Multicellularity was an advantage for plants, but the potentials seemed limited to floating mats or attached strands in the shallows.
Then the Snowball happened.
In the most extreme ice age ever, the planet virtually froze over, with even the tropical oceans iced down as much as a kilometer. No one knows for sure why (possibly from a particular formation of the barren continental plates), but for a very long time, the number of available places for living things became few and very far between. An intense period of competition for very limited resources might explain the next step: animals evolved multicellularity, and it was like the evolutionary floodgates opened up. After the Snowball thawed and the world could be recolonized, it seems to have become full of complex swimming and crawling eating machines. In a world where life had existed for 3 billion years, many types of complex multicelled animals "appear" in the fossil record over a period that might be as short as 40 million years, a period called the Cambrian Explosion. With maybe one exception, every known phylum of animals evolved during this period, including early members of our lineage.
The war was on. Plants seemed to be able
to deal with larger plant-eaters with few obvious
adaptations: algae remained relatively simple. But
animals got bigger and nastier, smaller and quicker, more
protected; think of an adaptation that provides an advantage
in a world of animals, and it appeared during the Cambrian
Explosion. Except one...
The land environment would have had several significant differences from the water environment that would need to be adapted to:
- NOTHING WAS ALIVE UP THERE. For the first, pioneer organisms (this term is applied to the newcomers in any "new" environment), they needed to deal with an environment devoid of life and nutrient-poor. Animals might use it as a place to avoid predators, but would need to return to the water to feed. Plants had a trickier obstacle: the light, water, and carbon dioxide they needed for photosynthesis might be available, but other nutrients for making molecules such as proteins would not be. It is quite likely that plants would not have been able to move onto the land without symbioses established with fungi and bacteria to help them get the materials they would ordinarily get from the water around them.
- WATER EVAPORATES IN THE AIR. The water content of cells is critical to the function of cells - if too much is lost or gained, the cells cease to function. A land organism cannot lose too much water to the air or it wont survive. But there are transitional ecosystems that might have required adaptations usable against evaporation: tidal zones, where organisms are sometimes left "high and dry," as well as in pools that might fill with rain runoff or evaporate, where resisting a similar dilution change in the cells would be necessary; fresh water systems, where a resistance to the inflow from very dilute surroundings would be necessary. Our distant ancestors, the bony fish, apparently evolved in fresh water and developed an efficient waterproofing system to keep water from rushing in; that barrier could also prevent water loss in the air. It is quite likely that, for this and other reasons, all life on land evolved from tidal and/or fresh water ancestors. Of the three multicelled Kingdoms, the fungi seem to have had the hardest time with drying, perhaps because of the way nutrients get absorbed - its almost impossible to move materials effectively across a waterproofed surface - but theyve gotten by in moister environments, in soils and in the wetness of other living things.
- YOU CANT FLOAT IN THE AIR. The buoyancy of water reduces the need for strong support structures. This was especially a problem for plants, which didnt undergo much dramatic evolution until they moved on to land, where the need for complex support structures and then structures to move materials around against the force of gravity led to an explosion of different forms. Animals had some adaptations ready to go: muscle systems for moving quickly through the water or across the bottom needed modification to work on land (fins needed to be more leg-like in our ancestors; insect and spider ancestors had to lighten their outer covering just to hold themselves up), but structures used for moving across tidal flats or in very shallow water became usable away from the water as well.
- TEMPERATURE FLUCTUATIONS. A body of water gains and loses heat more slowly than the air does, so temperature changes are slower there. Temperature has a huge effect on cellular chemistry, and only chemistry that can somehow deal with rapid changes can be used in a land organism. Again, tidal areas and shallow fresh water ecosystems would have been good staging areas for developing some flexibility. Plants, not being able to move from place to place to adjust their temperature, had a more critical problem, and may have taken some time to adapt to non-tropical areas.
- DIRECT SUNLIGHT. The frequencies of energy in sunlight can cause molecules in living systems to become unstable, as happens in the mutations that lead to skin cancer. Water reflects several frequencies and quickly absorbs many more, making the problem much reduced for organisms that live below the surface. Most land organisms have protective pigments to keep the sunlight from penetrating and harming them. The adaptations would also have been required for life in tidal areas and shallow fresh water.
- MUCH MORE OXYGEN. As mentioned earlier, the air can hold much more oxygen than water can, and oxygen is a very reactive material (you can be poisoned with too much of it!). An organism cant live in the air if it cant handle the increase in oxygen. Long-term, the higher oxygen levels allow for much more energetic metabolisms in aerobic animals. Even an animal like a crocodile gets such an energy advantage from breathing air that it would never evolve a water-breathing system again, and its difficult to understand how anyone could ever develop a system by which a human could breathe underwater - there just isnt enough oxygen.
- SPERM NEED WATER. Sexually- reproducing animals and plants had for the most part evolved systems where the swimming sperm were released and had to get themselves to the waiting egg cell. This doesnt seem like much, but for a couple of the major land groups it was the most difficult problem to solve - long after the difficulties of water loss, and support, and other land challenges were met, amphibians and ferns still required open water for reproduction.
Virtually every phylum of organisms was able to
get a least a few species up onto land, (some were there
long before the others, though) although they all still
have some water-living species as well. Some researchers
hypothesize that the rise of land plants, with hard-to-break-down
carbohydrate support structures, pulled more and more carbon from the
environment. Less carbon available for aerobic respiration might
have let more oxygen accumulate, setting up an environment for
higher-metabolism, larger animals.
Continental drift. The rocks that make up the continents mostly float like corks on heavier rocks beneath. These huge corks, called continental plates, move slowly but with huge momentum pulling away from each other to make things like the Atlantic Ocean or colliding in huge "fender benders" that ripple up things like the Himalayan Mountains or the Panama bridge between the Americas. These movement have huge effects on climate around the world, driving evolutionary change as areas get wetter or warmer or whatever.
Catastrophes. Sometimes the world can change in an instant. Huge chunks of rock fly in from space and smack into us, changing the weather for months or years and flash-frying whole continents. A low-lying basin connects to the ocean, and the Mediterranean Sea fills in seemingly overnight. A huge volcanic eruption covers almost a third of India with lava and spews huge amounts of climate-changing gas and dust into the atmosphere. Continental drift allows an invasion of new competitors into a stagnant ecosystem over a matter of decades. Major changes in climate, either warming or cooling, can affect almost all multicellular life. These can cause the major transitions found in the fossil record, including a few so large that they are known as mass extinction events; the asteroid impact that wiped out the dinosaurs and left our tiny scavenging ancestors to take over is a well-know one, but there have been several, and the causes are not always known. According to recent research, there may be a regular rise-and-fall of diversity (lots of different species, then a drop to very few, then a rise again) on a 62-million-year cycle; this may renew interest in an older theory about Nemesis, a proposed "dark star" companion of the Sun on a very long elliptical orbit - in the original hypothesis, it was supposed to visit every 26 million years, do bad things about the solar system with its gravity effects, and move away, but the theory could be amended to a 62-million-year cycle. Recently, the real nemesis may be closer to home: humans adjust the environment on a huge scale to fit their preferences, and this can be a problem for the organisms that share our environment.
One cause sometimes proposed is disease,
but this is very very unlikely as a major player for a couple of
reasons. For one, diseases tend to adapt to particular hosts
- a disease that can affect a wide variety of organisms is
extremely rare, and even then the effects vary because each type
of host is a unique ecosystem. More importantly, however, is
that diseases are caused by evolving organisms, and the more
successful individuals are not the nasty ones, but
the ones that keep the host semi-well and moving around to spread
its offspring. Except in tiny systems or small populations,
diseases get less damaging as they spread and so are unlikely
candidates for causing widespread carnage.
Online Introduction to Biology (Advanced)
Copyright 2003 - 2011, Michael McDarby.
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