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All life has evolved from a single cell, which has since developedinto more complex multicellular organisms over time. The biological complexityof an animal can be determined by a number of different characteristics.

 Multicellular organisms can be arranged into four differentlevels of organisation: cells, tissues, organs and organ systems. These rangefrom being the most simple to the most complex.  The cellular level of organisation includes cells, which arethe smallest functioning units of an organism, performing a specific function.Porifera are diverse and composed of a loose aggregation of cells. The celllayers of these sponges are not considered to be ‘true’ tissues as there is nobasement membrane or intercellular connections – the cells are relativelyunspecialised. Sponges are considered to be paraphyletic and to represent thelineage, which is closest to multicellular organisms.

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This suggests that otheranimals have evolved and shared a common evolutionary ancestor with sponges. The tissue level of organisation consists of a group ofcells which are similar in function that work together for a specific activity.The more complex an organism, the more distinctive tissue layers there are.Radiata are an example of a group of organisms which have attained this levelof organisation as their highest operating level. They are characterised bytheir radial symmetry – it has a top and bottom but no back or front.

 The organ level of organisation is composed of lots oftissues working together for a particular function. Apart from the Porifera andRadiata, most animals are triploblastic and so contain three germ layers – theectoderm, mesoderm, and endoderm. The organ system level of organisation is the highest levelbefore considering the organism as a whole. This consists of organs with acommon function working together.

Due to this all triploblastic organisms workat this level. Organ systems have been adapted to suit different types oforganisms. Fish have a single circulatory system where the blood travels fromthe heart to the gills and then to the rest of the body.

However mammals on theother hand have a double circulatory system. Blood flows from the heart to thelungs and back to the heart. The blood then goes straight to the rest of thebody with oxygenated blood. Flatworms are triploblastic but have the simplestversion of an organ system. They possess a number of different organs and thushave a high level of complexity. The evolution of biological complexity has been a majoradvancement for the process of evolution and has brought about more complex multicellularorganisms.

Specific levels of complexity are difficult to measure to anaccurate degree, as there are many different attributes: morphology, genecontent, and cell types are a few examples to name. Despite the lack of evidence there used to be a belief thatevolution was advancing and thus resulting in higher organisms. “Higherorganisms” refers to animals which have relatively developed characteristics. However this has been disregarded since organismsthat have been selected for, have either increased or decreased in complexityfollowing a change in the environment. Cyanobacteria were the pioneers of photosynthesis, whichtransformed the toxic atmosphere by producing oxygen and increasing it’s overallconcentration – this paved the way for the animals and plants of today.

Thechemical reactions using oxygen allowed for evolving animals to use new foodsources. Other organisms such as microbes died off in large numbers because theincreased concentration of oxygen was poisonous to them. However, cyanobacteriawere also the first cells to join forces and create multicellular life.

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