Have diets across the animal kingdom changed over time?

By Megan Widdows

We all know that what we eat is incred­ibly import­ant. It plays a huge role in our health, sus­cept­ib­il­ity to dis­ease and even our hap­pi­ness.  But why do we need to eat what we need to eat? 

For humans, a healthy diet con­sists of a nour­ish­ing bal­ance of car­bo­hydrates, pro­tein, fats, vit­am­ins, min­er­als, fibre and water. Each per­son needs a slightly dif­fer­ent amount of food, depend­ing on their unique energy expendit­ure. This depends on their size, meta­bol­ism and activ­ity levels. 

Each food group is broken down and diges­ted by our bod­ies in a dif­fer­ent way, requir­ing dif­fer­ent pro­teins, called enzymes. For example, amyl­ase breaks down starches and car­bo­hydrates into gluc­ose that is used as an energy source for our cells. Pro­teins are broken down by vari­ous pro­teases into their amino acid found­a­tions. Dif­fer­ent enzymes then join these amino acids back up to form new pro­teins that are used by the body to repair and build muscle. Sim­il­arly, lipases break fats down into gly­cer­ol and fatty acids. These are trans­por­ted to the liv­er before being sent to muscle cells to provide energy or, if there is excess, to adipose fat tis­sue for storage. 

But not all anim­als have the same diet­ary require­ments. In fact, across the anim­al king­dom there is huge vari­ation in foods that are eaten. Many anim­als, like us, are omni­vores, and are adap­ted to eat­ing a com­bin­a­tion of plants and meat-based foods. Big cats, such as lions, are one of many strict car­ni­vores. This means they only eat meat, and this provides all of their nutri­tion­al require­ments. Cows and sheep are herb­i­vores, which means they get all of their nutri­tion from grasses, seeds and grains. Their digest­ive sys­tem is even spe­cially adap­ted – their stom­ach has four sep­ar­ate com­part­ments each with a dif­fer­ent role, unlike our single com­part­ment that is bet­ter adap­ted to eat­ing a mixed, omni­vor­ous diet. 

These examples are both very dif­fer­ent from a healthy human diet that con­sists not only of six dif­fer­ent food groups (plus water) but also the requires vari­ety in the types of foods that make up these groups, to ensure we get all the vit­am­ins and min­er­als we need to function. 

The vari­ety of diet types seen across the anim­al king­dom led sci­ent­ists to ask how these diet­ary dif­fer­ences have evolved over time. To answer this ques­tion, research­ers stud­ied the diets of more than 1000 dif­fer­ent groups of anim­als (known as taxa) over a peri­od of more than 800 mil­lion years. They did this with a tech­nique known as phylo­gen­et­ic ana­lys­is, which allows sci­ent­ists to study the evol­u­tion­ary rela­tion­ships between spe­cies and how they have evolved, togeth­er and inde­pend­ently, through time. This inform­a­tion was com­piled with inform­a­tion about each spe­cies’ diet. 

A diet can be loosely grouped into one of the three cat­egor­ies covered above: car­ni­vor­ous, herb­i­vor­ous and omni­vor­ous. The sci­ent­ists found that across spe­cies, car­ni­vor­ous diets were most com­mon: 63% of the anim­al groups stud­ied were exclus­ive meat eat­ers. Herb­i­vor­ous diets were the second most com­mon, adop­ted by around 32% of all anim­al groups. Omni­vores, like most humans, are actu­ally very rare, mak­ing up just 3% of all the anim­al groups studied. 

As well as dis­cov­er­ing which diet is the most com­mon across anim­als, the sci­ent­ists also wanted to know how long these anim­al groups have been eat­ing the way they do now, and how did their diets evolve? 

The study revealed that the diets of dif­fer­ent anim­al groups have been main­tained for extraordin­ar­ily long peri­ods of time, remain­ing the same across mil­lions of years. This has allowed the evol­u­tion of spe­cial adapt­a­tions among anim­al groups to the food they eat. One clear example of this is the shape and size of teeth.

Car­ni­vor­ous anim­als have evolved pre­dom­in­antly large and sharp can­ine teeth that can be used to shred or rip flesh, mak­ing it easi­er to eat meat. Herb­i­vores have flat mol­ar teeth that are suit­ably shaped for crush­ing and grind­ing, which helps break down the tough plant mat­ter that they eat. The mixed diet of omni­vores has led them to evolve a com­bin­a­tion of can­ine and mol­ar teeth, allow­ing them to eat both plants and meat.

One import­ant find­ing of the study was the high fre­quency of car­ni­vor­ous spe­cies across the anim­al king­dom. This has led sci­ent­ists to believe that the com­mon ancest­or of all anim­als – the anim­al from which all mod­ern day anim­al spe­cies, includ­ing us, have evolved – was a car­ni­vore. This means that car­ni­vor­ous anim­als liv­ing today have prob­ably main­tained the same meat-eat­ing diet from ancest­or to ancest­or, on and on through their evol­u­tion­ary his­tory for more than 800 mil­lion years! This is an extraordin­ary find­ing and reminds us, yet again, of the fun­da­ment­al import­ance of diet to our bio­logy and health. 

Whilst anim­als seem to be largely stick­ing to their ancient diets, over the past few hun­dred years, there have been huge and rap­id changes to our own, human diets. We are increas­ingly eat­ing foods that are heav­ily pro­cessed and high in fat, salt and sug­ar that our bod­ies aren’t well adap­ted to. These types of foods were not avail­able, even to our recent ancest­ors, so we have not evolved to eat these types of foods. This is one pos­sible explan­a­tion for the recent surge in life­style dis­eases such as obesity, heart dis­ease and type 2 dia­betes. We, as oth­er anim­als, have evolved to eat a cer­tain diet, and these recent dis­rup­tions that have happened far quick­er than evol­u­tion can allow us to adapt.

The study men­tioned in this art­icle can be found here. A gloss­ary of terms is provided below.

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