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 in 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
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. Radial
symmetry is thus advantageous for stationary animals. All other organisms tend
to have a bilateral symmetry where there is only one plane. This promotes the
development of a head and a tail (posterior and anterior) and a back and front
(dorsal and ventral). Unlike radial symmetry, bilateral symmetry allows for
streamlined and directional movement. This allows for mobility to search for
resources to survive and also enables predators to seek prey, or for prey to
escape for safety.
The change from unicellular organisms to multicellular
organisms has evolved independently over a dozen times. The evolution started
from a single cell, which was focused on its own survival, to a multicellular
organism where all the cells coordinate and work together. Although some
Creationists may argue that this leap in evolution is so finely tuned that it
must have been with the help of a higher power, this has been disproven in the
lab – scientists have shown this by conducting experiments with single-celled
yeast evolving into a multicellular organism in the presence of a strong
Complexity can emerge from the co-evolution of hosts and
pathogens; each of which develops adaptations to survive against each other. An
example of this can be seen in the progression of the immune system: as
antibodies begin to remember and fight against specific antigens, pathogens can
develop methods to evade being destroyed. Trypanosoma
brucei is a parasite, which can cause sleeping sickness in humans. As so
many copies of its major surface antigen have evolved, a small proportion (10%)
of the genome is dedicated to different versions of this one gene. As a result,
the parasite can constantly change its surface and evade the immune system by
In conclusion, the growth of complexity of organisms can be
driven by many factors, with co-evolution having an important role. The
co-evolution between an organism and the ecosystem of the predators and prey,
which it tries to adapt to, has a large impact on complexity. As either of
these become more complex, to handle the diversity of threats in the ecosystem,
the others must also adapt and do so by becoming more complex. As a result this
triggers an on-going evolutionary arms race towards more complexity.