[thedoc (OP)]

Lee asked the Naked Scientists:
   
560 million years ago an organism called charnia was thriving. At that time it is my understanding that there were no  ambulatory or freely moving organisms.  Charnia was like a cross between an animal and a plant, rooted into the ocean floor... My question concerns the time in which life went from rooted nonfree moving organisms to free moving organisms. Using only common sense I can state that during one 10 thousand year stretch there were only nonfree moving/stationary organisms, and further in time in the next 10 thousand years we had free moving organisms.  However, when I narrow that time line it becomes harder to comprehend.  Indeed there had to of been a day when all life was stationary, then the next day they were by definition free moving. In fact there had to be a minute in which life went from one definition to another. However, everything I know tells me that evolution is a long process. That man going from the trees to walking upright took a vast interval of time. The problem is I can understand and visualize man walking upright being a gradual process.. charnia being stationary one minute and free moving the next I cannot... but then again, I'm not a scientist lol. Can you please help me understand this Sir?

Lee Yott, Michigan, U.S.

What do you think?

[evan_au]

Plants and animals have different stages in their life-cycle, some of which are more mobile than others. Having a mobile stage of life allows them to colonise new areas.

Some examples:

• Bacteria can crawl short distances, or free-float for longer distances. Flagella can provide some propulsion.
  
• Often the young have a free-floating stage
  
• I have seen some fossils from the Ediacaran period. Their body forms are so strange to us that it is hard to tell if some of them were mobile. Some fossils that look like the cast of a burrow or mud trails raise the possibility of mobile life-forms.
  
• Today's slime molds swap between a single-cell stage (like a bacterium) and a multicellular stage which is more mobile (it looks a bit like a slug). It is very unlikely that creatures like this would form fossils.

I would guess that the first animals and plants included free-floating mobile forms. More complex organisms probably retained a free-floating stage in their life-cycle, even if the adult form was rooted in one place. But a fossil record of these soft-bodied organisms is very elusive, and rather hard to interpret.

[CliffordK]

Indeed there had to of been a day when all life was stationary, then the next day they were by definition free moving. In fact there had to be a minute in which life went from one definition to another.

Humanity didn't evolve in a day, and neither did animals. 

One might ask, which came first plants or animals (which eat plants). 

It may well be that plants evolved from animals. 

Ok...  so it is not that animals devolved, but consider the major organelles in eukaryotes. 

For single celled eukaryotes, one has amoebas, and protozoa which have mitochondria for energy which is believed to have evolved through endosymbiosis, or the engulfing of bacteria. 

Plants also have mitochondria PLUS chloroplasts with the chloroplasts closely resembling cyanobacteria.  Many plants also have a secondary cell wall that is lacking in the animal kingdom.

So, it would be reasonable to conclude that the endosymbiosis of the mitochondria occurred before the endosymbiosis of the cyanobacteria (chloroplasts).  Then the evolution of multicellular plants and multicellular animals then progressed independently.  However, there are a few different types of chloroplasts, so perhaps the endosymbiosis  of cyanobacteria occurred multiple times. 

Many of the single celled eukaryotes are highly mobile with deformable membranes, flagella, and cilia.  It wouldn't be unexpected that some of them would retain mobility when they developed into multicellular organisms. 

One of the most important things required for the growth of large animals, however, was the release of free oxygen in what is known as the Great Oxygenation Event

Actually, there are a number of steps that must occur in the evolution of animals (some of which also occur in plants).  Another big development is meiosis which I don't believe occur in amoebas or protozoa, but can occur in fungi, as well as plants. 

So, perhaps the development is:

Single Celled Organisms --> non-photosynthesizing fungi --> animals
Single Celled Organisms --> non-photosynthesizing fungi --> lichens (fungi/cyano bacteria symbiosis) --> endosymbiosis --> plants.

Apparently fungi also produce chitin which is found in invertebrate shells.

Additional development in multicellular organisms would require cell specialization, as well as nutrient delivery to interior cells. 

Getting to something vaguely resembling animals would have taken many more steps, each developing somewhat independently.

• Eukaryotes and Mitochondria
• Multi Cellular
• Meiosis
• Cell specialization
• Skin
• Circulatory System.
• Some kind of a shell or skeleton
• Arms, Legs, Mouth, etc

Anyway, so it is not an overnight thing.  One day there wouldn't be a world without animals, and animals the next, but there are cumulative small changes leading to what we recognize as animals.

[dlorde]

Even today there is a huge range of creatures that have what we'd see as transitional forms and lifestyles - single-cell creatures that can form co-operative assemblies, motile larval stages with anchored mature stages, and so-on.

I can envision loose assemblies of single-cell creatures on tidal shores (rock pools, etc), that gain advantage by literally sticking together, that may occasionally be torn loose by wave action, forming free-floating colonies. Here there would be a dual selective pressure, in one direction for increased adhesion to the rocks, and on the other for survival when free-floating, which would be enhanced by some degree of control (as in cnidarians and ctenophores, i.e.'jellies') to rise & fall in the water column on a daily cycle (to find food and avoid predators).

Similar selection pressures would apply to static creatures loosely 'rooted' in shoreline sands where wave action might dislodge them - in one direction for deeper embedding, and in another direction for free-floating survival in the benthic zone.

In each case, some control of attitude and depth would be an advantage for the free-floating form, leading to differentiation and specialisation of cells for various purposes.

[cheryl j]

The interesting thing about chemotaxis in bacteria is that it has an objective - moving towards higher levels of nutrients and away from toxins. Clockwise rotation of flagella makes the bacterium twiddle, or change direction, and counter clockwise rotation causes it to move in a strait path. In order to move towards or away from a gradient, the bacteria has to have a way of recording and comparing two measurements, since it's not the absolute concentration that matters but the change in it. In other words, it has to have  a kind of memory, short as it may be, but a form of memory none the less.

http://en.wikipedia.org/wiki/Chemotaxis (see receptor regulation)
http://www.ncbi.nlm.nih.gov/pubmed/19747082

[CliffordK]

I will say that the evolutionary pressures on the organisms that photosynthesize, and those that don't may be extremely different.

A tree benefits from spreading its seeds far and wide, but derives very little benefit from moving to a sunnier location.  However, it does benefit from growing taller than its neighbors, and thus a strong root system.  Roots also give it access to nutrients, and strong roots prevent damage to the nutrient supply from nature, or herbivores (which, of course evolved later).

Decomposers, and herbivores benefit from getting to as many plants as possible, and thus mobility. 

So, while to a large extent animals evolved to move, plants evolved to stay in one place (only spreading their seeds far and wide).