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These notes explain how to run the ‘Highlighting Important Stages in Evolution’ activity ... - page 5 / 11





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These are thought to be closely related to sponges, consisting of only a few types of cells, some unique to the phylum; they do not have true nerve, digestion or blood systems. For these reasons, they are not direct ancestors of the more complex animals.


What is the significance of the characteristic shared by the group of animals that emerge at the point Y? Consider the relationship between this evolutionary development, the medium in which animals lived and the selection pressure favouring the characteristic.

This group of animals is called Bilateria. They possess bilateral symmetrical bodies that almost certainly evolved to provide a more streamlined body profile for faster and more efficient movement through water – the medium in which they first evolved. With the exception of the group known as the Echinoderms, most animals developing after this stage display bilateral symmetry. This stage of evolutionary development led to other significant evolutionary steps forward, including the emergence of a body plan based around three layers (triploblasty), the formation of a digestive tract with true mouth and anus and, ultimately, the migration of sense organs to the anterior (front) of the animal, and the formation of a distinct head.


One of the simplest members of this group does not possess a gut. It also has developed a symbiotic relationship with types of algae, which attach to it. Name this animal and suggest why this symbiotic relationship may have developed.

Acoel is a very primitive bilaterally symmetrical animal that does not have a gut cavity (the name means ‘without cavity’) Some members of this group have developed this symbiotic relationship with algae, since without specialised cells and with only cilia to bring about movement, they are limited in their mobility but are also able to derive sufficient energy from this symbiotic relationship with algae.



Velvet worms are descendants of some of the earliest forms of segmented animal. The appearance of animals with segmented bodies is considered a major development in the evolution of the animal kingdom. How did the appearance of repeating segments offer a significant evolutionary advantage for animals that evolved this body structure?

The early segmented animals would have had an advantage over competitors in that their movement was better coordinated and probably faster too. Velvet worms are adapted to squeeze into small spaces and exploit these locations for food.


Insects are the most diverse animals on Earth and belong to the phylum Arthropoda. Identify the earliest insect shown on the diagram and carefully circle the insect sub-classes shown. As it is the only class of invertebrate capable of flight, comment on the contribution made to insect success in possessing (i) a chitin exoskeleton, (ii) striated muscle tissue and (iii) highly developed sense organs.

The giant dragonfly is the earliest insect identified on the Tree of Life. The chitin exoskeleton could be moulded into many different shapes including structures associated with flight (wings etc). The striated or voluntary muscle was an evolutionary step forward in that it was able to contract and recover quickly, enabling the quick movement essential for flight. Highly developed sense organs enabled insects to detect quickly the changing events in their surrounding environment, especially important when flying.


Some of the early insects were much larger than they are today. One suggested reason for their greater size is that the atmospheric oxygen concentration was much higher at that time. Explain the significance of this hypothesis.

The insect system of gas exchange is effective but largely relies on diffusion of oxygen into the body of the insect. The rate of diffusion is limited by the partial pressure (or concentration) of oxygen in the atmosphere. During the Triassic period, there was a reduction in atmospheric oxygen concentration, which may have caused the giant insects and other large invertebrates to die out.

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