Question

1) What were the first animals like?

2) Compare the diversification of plants and animals. When do major body plans appear in these groups (for animals, think different phyla, and for plants, think algae/land plants/vascular plants/seed plants/angiosperms) and over how long a period of time do new plans appear)?

3) Why is bilateral symmetry strongly associated with cephalization, the concentration of the sense organs in an anterior ahead?

Answer 1

A) The evolutionary history of the comb jelly has revealed surprising clues about Earth's first animal.

Single-celled micro-organisms or microbes are the first living things on Earth those lacking a cell nucleus or cell membrane known as prokaryotes.

B)

A body plan  is "an assembled form of morphological features shared among many members of a phylum-level group".

Evolutionary developmental biology show the origins of diverse body plans. The vertebrate body plan is one of many: invertebrates consist of many phyla.

Body plans have historically been considered to have evolved in a flash in the Cambrian explosion, but a more nuanced understanding of animal evolution suggests gradual development of body plans throughout the early Palaeozoic.

The history of the discovery of body plans can be seen as a movement from a worldview centred on the vertebrates, to seeing the vertebrates as one body plan among many. Linnaeus identified two body plans outside the vertebrates; Cuvier identified three; and Haeckel had four, as well as the Protista with eight more, for a total of twelve. For comparison, the number of phyla recognised by modern zoologists has risen to 35.

The Swedish botanist Linnaeus grouped the animals into quadrupeds, birds, "amphibians (including tortoises, lizards and snakes), fish, "insects" (Insecta, in which he included arachnids, crustaceans and centipedes) and "worms" (Vermes).

The French zoologist Georges Cuvier combined evidence from comparative anatomy and palaeontology to divide the animal kingdom into four body plans. Taking the central nervous system as the main organ system which controlled all the others, such as the circulatory and digestive systems, Cuvier distinguished four body plans:

• I. with a brain and a spinal cord (surrounded by skeletal elements)
• II. with organs linked by nerve fibres
• III. with two longitudinal, ventral nerve cords linked by a band with two ganglia below the oesophagus
• IV. with a diffuse nervous system, not clearly discernible

Grouping animals with these body plans resulted in four branches: vertebrates, molluscs, articulata (including insects and annelids) and zoophytes or radiata.

Ernst Haeckel asserted that all living things were monophyletic (had a single evolutionary origin), being divided into plants, protista, and animals. His protista were divided into moneres, protoplasts, flagellates, diatoms, myxomycetes, myxocystodes, rhizopods, and sponges. His animals were divided into groups with distinct body plans: he named these phyla. Haeckel's animal phyla were coelenterates, echinoderms, and (following Cuvier) articulates, molluscs, and vertebrates.

Stephen J. Gould explored the idea that the different phyla could be perceived in terms of a Bauplan, illustrating their fixity. However, he later abandoned this idea in favor of punctuated equilibrium.

20 out of the 36 body plans originated in the Cambrian period, in the "Cambrian explosion", However, complete body plans of many phyla emerged much later, in the Palaeozoicor beyond.

The current range of body plans is far from exhaustive of the possible patterns for life: the Precambrian Ediacaran biota includes body plans that differ from any found in currently living organisms, even though the overall arrangement of unrelated modern taxa is quite similar. Thus the Cambrian explosion appears to have more or less completely replaced the earlier range of body plans.

According to Neo-Darwinism, new biological form arises when natural selection acts on randomly occurring mutations and variations in DNA. But new research seeems to say that mutations in DNA assembly instructions will produce, at best, a new protein. Higher-level instructions for building tissues, organs and body types are not stored only in DNA. This means that you can mutate DNA 'til the cows come home and you still wouldn't get a new body plan.

C)

Cephalization: is known as the presence of a head. Cephalization occurs in all bilaterally symmetrical animals because there is an advantage in having the end that goes first, as the animal moves, fitted out with sense organs on the premise that it is better to know where we are going than where we have been. Because a lot of computational power is needed to deal with sensory stimuli, the nervous system tends to be larger in the head as well. And finally, food can be captured more effectively if the mouth is also at the front of the body.