Kingdoms of Life

Copyright © Karl Dahlke, 2023

Carl Linnaeus, 1707 to 1778, was a Swedish botanist, zoologist, and physician. He spent most of his life cataloguing all the animals of the world, and even some that had gone extinct, building a comprehensive tree of life. The first division is between vertebrates and invertebrates - animals with spines an without spines. Vertebrates further subdivide into fish, amphibians, reptiles, birds, and mammals. Mammals further subdivide into three groups: monotremes, marsupials, and placentals. These further subdivide into various orders, genera, and finally species. His system was so insightful and powerful, that it remains largely intact today. We have moved a few animals about, using modern genetic information, but it's astounding how much Linnaeus got right.

Look up porcupine in wikipedia. It's an animal, that's pretty obvious, and it has a spine, so it is in the phylum cordata. The class is mammalia, and the order is rodentia. The porcupine is a rodent, like mice and rats and rabbits. Finally we get down to a couple of genera, comprising 30 species of porcupine.

Indeed, you can look up any animal in wikipedia, and note its position in the tree of life - a nod to Linnaeus 250 years later.

To the surprise of many, Linnaeus placed humans in with the primates. He said, “They basically have the same anatomy; except for speech.” Some considered this a sign of his genius, a true paradigm shift, but it ruffled quite a few feathers at the time. Almost everyone throughout recorded history believed man was a unique creation, separate from and superior to the animals. Genesis chapter 1 tells us God made all the animals on Thursday, then man on Friday. Surely we stand apart. Furthermore, we have dominion over the animals, as affirmed in verses 26 through 28. This certainly strokes our ego, does it not? But Linnaeus said no. He put his ego aside and followed the science; based on anatomy and physiology, we are just another primate, like a monkey or an ape.

Linnaeus wasn't the first to place man on an equal footing with our furry friends. Note these verses from Ecclesiastes chapter 3, written some 3,000 years earlier. Solomon, the presumed author of Ecclesiastes, was indeed wise.

“Man's fate is like that of the animals; the same fate awaits them both: As one dies, so dies the other. All have the same breath; man has no advantage over the animal. … All go to the same place; all come from dust, and to dust all return. Who knows if the spirit of man rises upward and if the spirit of the animal goes down into the earth?”

Switching to invertebrates, let's see where the grasshopper lies in the tree of life. It is an animal of course, and its phylum is Arthropoda, which is Latin for jointed feet. The class is insecta, since it has 6 legs. Other arthropods are spiders, crabs, and centipedes, along other branches of the tree. These have 8, 10, and many legs respectively. Returning to insects, the grasshopper's order is Orthoptera, based upon its straight wings. In an earlier article, I explored various insect orders, as determined by common physical features, primarily the wings. Continue down the grasshopper branch, and find at least 11 thousand species. Obviously these were not all catalogued by Linnaeus. We have added thousands of species to his tree throughout the years, especially in the insect world, but his framework remains intact.

Apparently this staggering accomplishment wasn't sufficient; Linnaeus also classified the plants. Look up tulip in wikipedia, and find the following.

Kingdom: Plantae
Clade: Tracheophytes (vascular)
Clade: Angiosperms (flowering)
Clade: Monocots (single embryonic leaf)
Order: Liliales
Family: Liliaceae
Subfamily: Lilioideae
Tribe: Lilieae
Genus: Tulipa

There are about 75 species of tulip. I wouldn't be surprised if Linnaeus was familiar with most of them. He loved plants, and published is plant classifications before the animals.

With regard to foods, plant families can be surprising, since we often assess differences and similarities based on the edible portion that sits upon our plate, rather than the entire plant itself. For example, the white potato, the sweet potato, and the yam all belong to different families, nightshade, morning glory, and dioscoreaceae, respectively. And what other foods belong to the nightshade family, along with the potato? Tomatoes, peppers, eggplants, and tobacco. 🥔🍅🌶🍆

Linnaeus established two separate kingdoms: plant, and animal. This is rather intuitive. Plants are rooted to the ground, and derive their energy from sunlight. Animals run about, (or swim or fly), and eat plants, or other animals who ultimately eat plants. The higher animals even have personalities. Indeed, the word animal comes from Latin animalis,possessing breath or spirit. This also gives us the words animate and animation, to give life to something.

Employ a microscope, and you'll see another difference. A plant cell contains green spheres called chloroplasts. These hold chlorophyll, the magic compound that turns sunlight into energy. This is what makes plants green. Another characteristic of a plant cell is its cell wall, which gives plants strength and stability. An animal cell has a membrane, (every cell has to have a barrier separating its delicate chemistry from the environment), but a membrane can be pliable, like a soap bubble, whereas a cell wall is rigid. These cells stack up to make a stalk of corn, waving in the wind, or even a towering tree. Therefore, plants and animals are quite different, even at the cellular level. Linnaeus had a microscope, and was well aware of this distinction. This provided further evidence for his two kingdoms, plants and animals.

They are so different, one wonders if they evolved separately. we can't even estimate the odds of life appearing on a copasetic planet; maybe it evolved twice on earth. Such a question would never have been expressed in these terms in 1760, a century before Darwin's pivotal publication, On the Origin of species. Rather, the question might have been asked, “Are plants and animals separate acts of creation by God?” No matter how you frame it, our intuition says no. Plants are biochemically compatible with us, in fact they meet all of our nutritional needs. This would be a bit surprising if the kingdoms were separate acts of creation (by God), or rolls of the dice (by evolution). Turning this on its head, some plants capture and digest insects, from the animal world. The venus flytrap is well known, but there are other examples, such as the pitcher plant, and the sundew genus, comprising 152 carnivorous species.

Modern chemistry affirms our intuition. I'll just give one example. In our DNA, the triplet GCT codes for the amino acid alanine. This encoding is arbitrary. AGA could represent alanine just as well. However, it's the same code across all the animals, and all the plants.

Take a step back, and be amazed that plants use DNA at all, the familiar bases A G C and T, to store and pass on genetic information, just as we do. They could use another mechanism entirely, but they don't. Furthermore, the codings are all the same, as if plants and animals had read from the same book. Therefore, both kingdoms have a common ancestor, billions of years in the past. Linnaeus couldn't imagine such an ancestor, nor the time frames involved, nor the mechanism that produced his tree; that would come later.


What is a species anyways? We toss the word around casually, but what does it mean?

A species is a group of animals who can and do mate, and produce viable offspring. It's a bit more complicated than that, but that is the definition in one sentence.

Dogs and cats are different species, because they can't mate.

Dogs are one species because they can and do mate, but there are some caveats. A big dog can mate with a medium dog, and a medium dog can mate with a smaller dog, but the big dog cannot mate with the small dog. Still, the doggy genes mix throughout the generations. If a gene appears as a mutation, and it is advantageous, it will make its way throughout the entire dog population in a hundred generations.

A horse and a donkey can mate; the offspring is a mule. However, the mule is sterile; that is the end of the line. There is no mule species, nor are mules the catalyst that would merge horses and donkeys together.

Tigers and lions can mate, but for various reasons, they don't. When we convince them to mate, usually in captivity, the child is a tigon when the father is a tiger, and a liger when the father is a lion. These can interbreed further, for example, a lion and a liger producing a liliger. However, in the wild they remain in separate realms, and represent different species.

All humans are one species, since we can and do interbreed.


There is another kingdom, seen only through a microscope. These are bacteria, which are everywhere. Nobody paid much attention to them in the late 18th century; they didn't seem very interesting. We didn't associate them with infection and disease until the late 19th century, and we didn't understand their health benefits, in a symbiotic relationship, until the late 20th century. People saw the little critters, then put the microscope away, and tried to pretend they weren't swimming about in their drinking water. “Just put it out of your mind.”

How do we know this is a third kingdom? Perhaps bacteria are just free floating plant or animal cells that haven't come together to make a cooperative structure. Well they don't have green chloroplasts or a cell wall, so they aren't plants. Turns out they aren't animal cells either. Plant and animal cells are much larger, having 20 times the diameter and ten thousand times the volume of a bacteria. Also, plant and animal cells have a nucleus, which contains our DNA and other vital structures; bacteria do not. (They hardly have room for a nucleus.) This fundamental difference places them in a third kingdom.

Is it an unrelated kingdom, a separate act of creation? No, because it has the same biochemistry inside. Bacteria store genetic information in DNA, just like we do, and GCT still codes for alanine. This holds true across all the kingdoms of life. They all come from one common ancestor. Life evolved once on earth, almost 4 billion years ago, and diverged from there.


there is a fourth kingdom, the kingdom of fungi, though these were still considered plants in the 1700's. Indeed, mushrooms grow out of the ground like plants, and do not move about, and some of them are edible, providing a valuable food source. They certainly look like plants. However, we now place fungi in their own kingdom, which diverged from plants and animals a billion years ago.

A fungal cell has a nucleus; thus it is not a bacteria. It has a cell wall, but this wall contains chitin, rather than cellulose. It has no chloroplasts, and cannot harvest sunlight for energy. The macroscopic structures that fungi sometimes produce, such as mushrooms, look like plants, but they are not. Nor are they animals, for various biochemical and morphological reasons.

The fungal kingdom is not as well known as plants and animals; here are some examples.

When confronted with an infection, it is important to determine which kingdom is involved. Antibiotics, which kill bacteria, have no effect on fungi, and conversely, fungicides do not combat bacteria. Neosporin (antibacterial) won't do anything for diaper rash, and Lotrimin (antifungal) won't cure an infected cut. Your doctor will determine the type of infection and the appropriate treatment. Nothing in this book should be construed as medical advice.

It is extremely rare, but scientists have recently discovered a fungi that eats animals. Deep in eastern Brazil's Atlantic Forest, a team of biologists spotted a fuzzy purple stalk protruding from the leaf litter on the ground. Following the spore-covered body down into the soil, they found a mummified spider swaddled in fungal filaments called hyphae. Once again, this confirms a compatible biochemistry across kingdoms. Read more here.

Other Kingdoms

Over the past 50 years, scientists have revised the definition of kingdom several times, as it relates to microscopic organisms. (This does not affect the familiar kingdoms of animals, plants, and fungi.) Kingdoms have ranged from 5 to 8. As of 2015, we have settled on 7 kingdoms under 2 empires. The first empire arose billions of years ago, small cells without a nucleus. The second empire, more recent, comprises the nucleated cells.

Prokaryota [ bacteria, archaea].

Eukaryota [protozoa (Latin for early animal, and still microscopic), chromista (brown algae, seaweed, and kelp), plantae, fungi, animalia].