THE EVOLUTION OF LIFE

LEARNING OUTCOMES

  • Define phylogenetic trees and taxonomy
  • Understand the science behind phylogenetic trees and taxonomy, and explain how they work
  • Recall the “father of taxonomy” – Carl Linnaeus
  • Define the Tree of Life
  • Outline the full taxonomic classification of humans, from domain to species

INTRODUCTION

So we know that evolution tells us where the variety of life comes from, and why creatures are so perfectly adapted to their habitats. However, it also tells us how all life on Earth share a common ancestor, just like you and your cousins share a common grandfather. Life on Earth began more than 3 ½ billion years ago, from the first single-celled organism to all the diverse life forms we see today. We, as humans, are therefore distant cousins with all living things, even plants!

Watch this video by AsapSCIENCE to see what Earth’s 4.5-billion-year timeline would look like if it was compressed into a day, and how all life on Earth only started at 4am:

 

But how do we keep track of all these different kinds of life, living and extinct? How do we know which creatures are more closely related and which are more distant? Phylogenetic trees and taxonomy.

PHYLOGENETIC TREES AND TAXONOMY

To stay organised, biologists use phylogenetic trees (also known as evolutionary trees) and taxonomy. 

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A phylogenetic tree is a branching diagram used to explore the evolution of life, show the evolutionary history (ancestry) of species and describe the relationships between species, living and extinct.

Understanding a phylogenetic is a lot like reading a family tree. The root of the tree represents the ancestral lineage, and the tips of the branches represent the descendants of that ancestor. As you move from the root to the tips, you are moving forward in time.

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When a lineage splits (speciation), it is represented as branching on a phylogeny. When a speciation event occurs, a single ancestral lineage gives rise to two or more daughter lineages.

Phylogenies trace patterns of shared ancestry between lineages. Each lineage has a part of its history that is unique to it alone and parts that are shared with other lineages.

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Similarly, each lineage has ancestors that are unique to that lineage and ancestors that are shared with other lineages — common ancestors.

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Taxonomy is based on these evolutionary relationships among organisms and organises species in ways that reflect our understanding of how they evolved from their common ancestors. It is defined as the process of naming and classifying organisms into groups within a larger system, according to their similarities and differences. Biologists currently use the “Modern Taxonomic Classification System”, where organisms are put into 8 taxonomic groups that indicate their natural relationships. It is based off of Carl Linnaeus’s original taxonomic system, who is regarded as the “father of taxonomy”.

 

THE TREE OF LIFE

The Tree of Life is the largest phylogenetic tree and includes all the living and extinct species on Earth (that we’ve discovered). It illustrates how all organisms arose from the last common universal ancestor via evolution, and that all of life is connected. 

 
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The Tree of Life basically tells us how related species are to everything else, however recently or distantly. Like a tree, the Tree of Life has many branches, where different species evolved from a common ancestor. At the end of all the branches are all the different individual species. The more recently two branches come together (converge), the more recent their common ancestor, and the more closely related they are.

As you continue to go down the tree, species become less and less related. For example, humans and chimpanzees, who are very close to one another, diverged from a common ancestor only a couple of million years ago, whereas humans and plants are much more distantly related, diverging hundreds of millions of years ago. If we get all the way to the bottom of the Tree of Life, we get to the trunk. The trunk of this tree marks the beginning of life on earth, a single-celled organism, the common ancestor that every organism is descended from.

At the most basic level, the first level of the system, are three Domains of life – Bacteria, Archaea and Eukarya. Bacteria and Archaea are unicellular prokaryotes (single-celled organisms that lacks a nucleus and membrane-bound organelles), whereas Eukarya contains all eukaryotes (multi-celled organisms that contain cells with a membrane-bound nucleus and organelles).

Within the Domain Eukarya, there are four Kingdoms – Animalia, Plantae, Fungi and Protista. Animalia contains all animals, Plantae includes all plants, Fungi contains all mushrooms and Protista contains all the rest (though many biologists don’t recognise Protista as a valid Kingdom anymore).

Within each kingdom, there are several Phyla (singularly known as Phylum). Within the animal kingdom, there are nine major ones. These include the Phyla Arthropoda, which contains all kinds of bugs, lobsters, creepy crawlies. Mollusca includes octopi, snails etc. Then there is Chordata. This Phylum includes any animal with a spinal cord (vertebrates). The divergence of vertebrates and invertebrates was a major step in evolution.

Within each Phylum, we can find different Classes of organisms. Some of the classes in the Phylum Chordata are: Reptilia, which are reptiles like crocodiles, snakes and lizards; Amphibia, which are amphibians like toads and frogs; and Mammalia, which are all the mammals, from kangaroos to elephants.

 
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Humans belong to the Class Mammalia, and within this Class there are 16 Orders. The Order Cetacea contains all the whales and dolphins. Carnivora contains organisms like bears and weasels. Then there are the Primates, which includes orangutans, lemurs, bonobos, and us humans too.

Within the Order of Primates, there are several Families. One of them is the Family Hominidae, which contains all the great apes, including us (we are a great ape too!).

Within the Family Hominidae, every organism here belongs to a particular Genus. This is where we diverge from the apes. Our Genus is called Homo, which means man in Latin. Within a Genus, there are several Species. All the species within the Genus Homo are now extinct, except for us, Sapiens. The species that came before us, including Australopithecus, Homo Habilis, Homo Erectus and many others are now all gone.

So every species can be classified by this taxonomic classification based on the evolutionary (phylogenetic) tree of life.

For example, the taxonomic classification of humans is:

  • Domain – Eukarya
  • Kingdom – Animalia
  • Phylum – Chordata
  • Class – Mammalia
  • Order – Primates
  • Family – Hominidae
  • Genus – Homo
  • Species - Sapiens

To help people understand our enormous family tree and the phylogenetics behind it, NOVA Labs have created an amazing resource called the NOVA Evolution Lab. In this lab, you can explore the “Deep Tree”, probably the largest version of the tree of life ever made (over 70,000 species)! You can also play the game “Build A Tree”, where you create phylogenetic trees from a set of species and a list of shared traits.

 
Screenshot of  NOVA Evolution Lab  by NOVA Labs

Screenshot of NOVA Evolution Lab by NOVA Labs