All life has evolved from a single cell, which has since developed
into more complex multicellular organisms over time. The biological complexity
of an animal can be determined by a number of different characteristics.
Multicellular organisms can be arranged into four different
levels of organisation: cells, tissues, organs and organ systems. These range
from being the most simple to the most complex.
The cellular level of organisation includes cells, which are
the smallest functioning units of an organism, performing a specific function.
Porifera are diverse and composed of a loose aggregation of cells. The cell
layers of these sponges are not considered to be ‘true’ tissues as there is no
basement membrane or intercellular connections – the cells are relatively
unspecialised. Sponges are considered to be paraphyletic and to represent the
lineage, which is closest to multicellular organisms. This suggests that other
animals have evolved and shared a common evolutionary ancestor with sponges.
The tissue level of organisation consists of a group of
cells which are similar in function that work together for a specific activity.
The higher the complexity of an organism, the more distinctive tissue layers it
has. Radiata are an example of a group of organisms which have attained this
level of organisation as their highest operating level. They are characterised
by their radial symmetry – it has a top and bottom but no back or front.
The organ level of organisation is composed of lots of
tissues working together for a particular function. Apart from the Porifera and
Radiata, most animals are triploblastic and so contain three germ layers – the
ectoderm, mesoderm, and endoderm.
The organ system level of organisation is the highest level
before considering the organism as a whole. This consists of organs with a
common function working together. Due to this all triploblastic organisms work
at this level. Organ systems have been adapted to suit different types of
organisms. Fish have a single circulatory system where the blood travels from
the heart to the gills and then to the rest of the body. However mammals on the
other hand have a double circulatory system. Blood flows from the heart to the
lungs and back to the heart. The blood then goes straight to the rest of the
body with oxygenated blood. Flatworms are triploblastic but have the simplest
version of an organ system. They possess a number of different organs and thus
have a high level of complexity.
The evolution of biological complexity has been a major
advancement for the process of evolution and has brought about more complex multicellular
organisms. Specific levels of complexity are difficult to measure to an
accurate degree, as there are many different attributes: morphology, gene
content, and cell types are a few examples to name.
Despite the lack of evidence there used to be a belief that
evolution was advancing and thus resulting in higher organisms. “Higher
organisms” refers to animals which have relatively developed characteristics. However
this has been disregarded since organisms that have been selected for, have
either increased or decreased in complexity following a change in the
Cyanobacteria were the pioneers of photosynthesis, and are photoautotrophs,
which converted the toxic atmosphere by producing oxygen and increasing its
overall concentration – this paved the way for the animals and plants of today.
The chemical reactions requiring oxygen allowed evolving animals to use new
food sources. Other organisms such as microbes died off in large numbers
because the concentration of oxygen had increased to such a level that it was
poisonous to them. However, cyanobacteria were also the first cells to join forces
and create multicellular life having evolved independently in many lineages.
Numerous features of increasing complexity can differentiate
the phylum of organisms: an example of this is body symmetry. This describes
how the body parts of an organism align around a central axis. Porifera are the
most primitive of animals and lack body symmetry – they are thus asymmetrical.
This is due to their lack of ‘true’ tissues and organs. Cnidaria have a radial
symmetry (they have a top and a bottom with an oral and aboral side) and so
experience the environment from all sides equally. This is especially suitable
for sessile animals as their sensory receptors are thus evenly distributed. All
other organisms tend to have a bilateral symmetry where there is only one