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The Co-Evolution of Metabolism and the Immune System   
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Evolution of Metabolism

In the obelia above, digestion of food starts in the digestive cavity, outside of each individual cell!

Over time more complex organisms, with more and more cells, and a greater need for energy, appeared.

Extra Cellular Digestion
Some cells formed stomach-like pockets, which became special areas devoted to eating, like a kind of stomach.

In the obelia, to the right, we can see a special pocket or cavity. This pockets is lined with special cells including swallowing cells that release tiny chemicals to the pocket. These chemicals breakdown food as itcomes in.

These different cells cooperate to breakdown and absorb the food, and then share the food energy with their neighboring cells. So, digestion started to occur more outside each cell, in the digestive pocket - or, extra-cellular digestion.

The Obelia shows a stomach-like pocket lined in eating cells. These cells digest the food and then easily share the energy with their neighbors.

Distribution System
As animals got larger, some cells became further and further away from the eating cells—and wouldn’t be able to get food in the simple way. So in larger animals, cells are found to have formed passageways, like human blood vessels, that spanned their whole bodies, to distribute energy and oxygen to all their cells.

In the polychaete above, passageways like blood vessels transport food and oxygen.

The circulatory system of blood vessels is not the only distribution system in the human body! Our immune cells also travel in the lymphatic system, a super immune system of vessels and nodes that span the body.

The more an animal can eat, the faster and the more efficiently he can digest its food, the more energy an animal has to move around, and seek food and a confortable environment.

In order to eat faster and get even more energy, some animals’ stomach pocket elongated to become a passageway going through the center of their entire bodies—the gut or digestive tube!  This digestive tube allows the outside environment—the ocean and all its nutrients—to pass through the animal without being in contact with most of its cells

Other special pockets evolved around the digestive tube in humans, forming organs like the liver and the pancreas. The pancreas helps by making chemicals that help break down food in the intestine. Also, it makes insulin that helps cells absorb sugars really well.

The digestive tube, being very good at absorbing broken down food, is also a big point of entry for outsiders! So, lots of immune cells are found very close to digestive tubes. 

As organisms became more complex and specialized, with many different types of cells, the harder the immune system’s job became. Think of the human body, how many different cells and particles can you think of that belong to you body? Skin cells, hair cells, eye cells, calcium in your teeth, bone cells!.. So, the immune system needed to become more complex! New types of cells, like B cells and T cells, and tissues, like the lymphatic system, evolved to work together and be more efficient and specific. Once the immune system has discovered a particle or dead cell in your body, it calls a team of special swallowing cells that set to work breaking it down, like food!

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