The water is a great place for a living thing,
since they depend on water to float their molecules and support
their chemistry. Life evolved in the oceans and filled them
wherever there was enough light to photosynthesize or food to
eat. But there were niches going unused, up out of the
water, on the bare land (the terrestrial ecosystems)
of the continents. How could it be
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
nitrogen-fixing fungi and
bacteria to help them get the materials they would ordinarily get from the
water around them.
Terrestrial vs aquatic food webs.
Symbionts and plants
A transitional vertebrate.
- 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 won't
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 cellular 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 - it's almost impossible to move
materials effectively across a waterproofed surface - but they've gotten by in
moister environments, in soils and in the wetness of other living
Tidal pools as
More on the transition.
terrestrial groups way back when.
The transition and water regulation
- YOU CAN'T FLOAT IN THE AIR. The
buoyancy of water reduces the need for strong support
structures. This was especially a problem for plants, which
didn't 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
Terrestrial plant adaptations.
Terrestrial plane evolution (pdf).
- 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.
Some temperature records (Montana).
getting more extreme?
- 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.
Carotenoids as light-effect protection (abstract).
- 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 can't live in the air if it can't
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 it's difficult to understand how
anyone could ever develop a system by which a human could breathe
underwater - there just isn't enough oxygen.
Effects of varying oxygen levels.
Oxygen levels and historic insect size.
- 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 doesn't 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.
Land animal groups - when? (student blog post)
How plant structures might have led to increased oxygen.