Talking Bio with UCSB Professor Alice Alldredge
For this edition of Curioser and Curioser, Martha Sadler sent questions via email to UCSB professor Alice Alldredge (pictured), who is the chair of the Department of Ecology, Evolution, and Marine Biology and teaches the animal kingdom portion of the university’s introductory biology series. The professor, a marine biologist who personally studies zooplankton (also pictured below), explained that “these were odd questions” but managed to entertain our queries successfully. So here we go.
1) Is there any sense to the notion that animals are mutated and evolved forms of plant, that they retain some plant characteristics?
Animals and plants form two major kingdoms in the classification of organisms. Both kingdoms arose from one-celled protists. Keep in mind that the ability to photosynthesize does not define whether something is a plant or not. Many algae, which are protists, can photosynthesize and are not classified as plants. The protists that are ancestral to plants had the ability to photosynthesize while those that are directly ancestral to animals did not. However, plants and animals do share a protistan ancestor from very long ago. Thus they share many cellular characteristics and actually have some common genes.
But animals are not mutated or evolved forms of plants.
2) Is it ever hard to draw the line between plant and animal?
No, it is not difficult to draw the line.
Plants are defined as organisms that: 1) Develop from embryos that are protected by the tissue of the parent plant (for this reason plants are sometimes called embryophytes);
2) Use chlorophylls a and b for photosynthesis;
and 3) Use starch as a photosynthetic storage product. (Note that some algae have the second characteristics, but not the first one, so are not plants.)
Animals are defined as being:
1) multicellular, with tissues usually differentiated into organ systems or cell layers;
2) receive their nutrition from organic molecules they obtain from the environment (like protein, carbohydrates, fats) .They cannot make organic molecules from inorganic ones the way plants do but instead ingest these organic molecules by eating other organisms or absorbing them through their body walls;
and 3) Show some capability of locomotion and sensory response at some point in their life cycle. With only very few exceptions, they have some kind of a nervous system that relays signals throughout the body electrochemically.
3) Would life on another planet necessarily be cellular?
Well, in that carbon-based life as we know it must depend on the interactions of chemical molecules, and it helps to have those molecules held together in some kind of a concentrated environment for them to work together, then yes, I think life on other planets is very likely to be cellular. But a cell might need to be very broadly defined: a sack holding chemicals together. Having never been to another planet where there is life, this is an educated guess!
4) Is it tricky to draw the line between lifeform and non-lifeform?
No. Living organisms are cellular and contain genetic material known as nucleic acids DNA, RNA, etc. They have the important characteristic of being able to reproduce and pass information to the next generation. Viruses are tiny infectious particles consisting of a core of nucleic acid (genetic material) surrounded by a coat of protein that can reproduce within the cells of living organisms. They are not considered living themselves because they are not cellular.
5) Plants are responsive to their environment, right? Leaves seek sun, roots seek water; plants heal their wounds. Is there any possibility they are operating on a pleasure principle when they reach for the sun? Or a pain principle, or that they experience longing or desire or will or something analogous to dreams? Or is it all purely mechanical? (Good luck with this weird question.)
“Feeling” implies a nervous system that can respond to the environment. Plants don’t experience pleasure or pain because they have no nervous system to detect these. Certainly they grow toward light or moisture, but this is purely mechanical. Words like longing, desire, will, and dreaming again imply a complex nervous system.
6) How did life begin?
The first cells probably assembled from organic molecules. Assemblages of “protobionts” which are assemblages of non-living organic polymers have been created by scientists in the laboratory. Some of these assemblages resemble living cells. For instance, some divide when they get too big. Others produce electrical potential across their surfaces and adsorb materials from their surroundings. These are things cells can do. These give us clues. The first cells were probably simple bags of chemicals surrounded by a porous organic membrane.
How organic molecules got concentrated enough to form protobionts is controversial. Some scientists think this could have occurred in hydrothermal vents in the deep ocean floor. Others think organic molecules formed on the earths surface in its early history and accumulated in shallow seas.
7) Was the first lifeform a plant?
No, certainly not! Plants are highly complex organisms that evolved long after the first cells. The first living things were very simple cells that lacked a nucleus and probably absorbed organic molecules from the soup around them. They could not photosynthesize.
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