The Tree of Life is a visual representation of how all life on earth is related – bacteria, archaea, protists, fungi, plants and animals.
The Tree of Life is generally shown as a tree-like diagram where the common ancestor of all life is at the base of the tree and each branch presents a group of organisms that share a common ancestor.
Various trees have been proposed over the centuries since Darwin first mooted the idea. Until the 1960s it was thought that the tree would have five main branches, each one representing a Kingdom (such as Plant or Animal) but when genetic sequencing became available on a large scale it became clear that very early in life on earth there were three separate lines of descent from a common ancestor. In the new tree all organisms were placed in one of three Domains (Eukarya, Archaea or Bacteria). This is the basis for the simplified tree shown in this diagram. Click on any diagram to enlarge it.
The chart below shows some of the Geological and Evolutionary Events that have taken place since the formation of the earth. Note that all times are given in million years ago, abbreviated to mya. These times are based on the fossil record, which is incomplete, and more recently on genetic analysis of existing organisms. They are, therefore estimates and subject to change.
Life on earth began about 3.7 billion years ago. The first prokaryotic single cells appeared 3.5 billion years ago. For over 1 billion years they existed before eukaryotic cells emerged. Prokaryotic cells are simple in structure with no membrane-bound organelles and their DNA is located in a region called the nucleoid which is not surrounded by a membrane. They exist today in the domains Bacteria and Archaea. Eukaryotic cells, in contrast, are much larger and have specialised structures such as a nucleus (which hold the cell’s DNA), mitochondria (which generate energy) and chloroplasts (which carry out photosynthesis in plants). Eukaryotic cells exist today in the four Kingdoms – Animals, Plants, Fungi and Protists. Prokaryotic cells are 10 to 20 times smaller than eukaryotic cells and much simpler. They depend on diffusion to move nutrients and wastes around and this limits their size. In eukaryotic cells their more complex processing is carried out within specialised structures.
Bacteria are single-celled prokaryotic microorganisms found everywhere on earth. Some are beneficial and some cause disease. They cannot form multi-cellular organisms but can form aggregates or colonies when multiple cells are attached to one another or to a flat surface by adhesive polymers. Bacteria are often classified by shape which can be spherical (cocci) e.g. Streptococcus, rod-shaped (bacilli) e.g. E. coli, spiral (spirilla) e.g S.volutans, comma (vibrios) e.g.V cholerae or corkscrew (spirochaetes). e.g. T palladium. Bacteria are very small microorganisms that are typically a few microns in size and can only be seen with the aid of a microscope. A micron is one thousandth of a millimetre. Viruses are even smaller at 0.1 micron or less and plant cells much bigger at 100 micron, nearly the same size as the thickness of a human hair. Most bacteria reproduce asexually by binary fission. This produces clones with identical DNA and allows bacteria to divide quickly – populations can double in 20 minutes given the right conditions. Some bacteria can reproduce sexually, exchange genetic material by means of a temporary bridge and acquire new traits such as antibiotic resistance. Bacterial cell walls are made of a polymer (peptidoglycan), animal cells have a plasma membrane so no wall, plant cells are cellulose and fungi cells chitin. The antibiotic penicillin interferes with the synthesis of the bacterial polymer.
Archaea are single-celled prokaryotic microorganisms and are similar to, but evolutionarily distinct from, bacteria. They differ in their DNA and the way they produce proteins and appear to be more similar to eukaryotes than bacteria. Some believe that they may be our direct ancestors and that there are only two branches of life – eukarya and bacteria. Archaea have been around since the beginning of life on earth and have been found living in extreme environments when they are known as extremophiles. Their cell wall differs in structure from that of bacteria and is thought to be more stable in extreme conditions. Some are found in hydrothermal vents on the ocean floor where they can withstand high pressures and temperatures. Some live in the guts of humans and ruminant animals where they must survive low oxygen levels and some live in salt pans or acidic sulphur springs.
Eukarya. This domain includes all the single and multi-celled eukaryotic organisms that comprise the animal, plant, fungus, and protist Kingdoms. Eukaryotic cells developed 2.0 billion years ago, long after the evolution of the prokaryotic cells of archaea and bacteria. A crucial phase in their evolution was the acquisition of membrane – bound organelles by absorbing prokaryotic cells. Over a long period of time these became symbiotic partners. In this diagram a eukaryotic animal cell is described, it has no chloroplasts. In eukaryotic cells DNA is held and transcribed in the membrane-bound nucleus. The resulting messenger RNA exits the nucleus via pores which also allow ribosomes produced within the nucleolus to leave. Ribosomes, located on the surface of the Rough Endoplasmic Reticulum read the mRNA and synthesise proteins from amino acids. The Smooth Endoplasmic Reticulum synthesises lipids such as steroids. The Golgi apparatus is an organelle that modifies, sorts, and transports proteins and lipids within the cell. Mitochondria break down organic molecules to produce energy in the form of ATP. Lysosomes break down and recycle unwanted materials. Peroxisomes break down fatty acids and amino acids and harmful substances and Vacuoles are storage vessels. Not shown is the cytoskeleton, a network of protein filaments and microtubules that keep the cells shape and internal organisation. It is important in fixing or allowing organelles to move and in cell division.
Protista. This kingdom includes all eukaryotic organisms that are not classified in the animal, plant or fungus kingdoms. They can be unicellular or multicellular and have diverse forms and functions but are relatively simple. They have the typical membrane-bound organelles of the eukaryotic cell as shown in the attached drawing of a Euglena unicellular organism. Multicellular forms such as Kelp, shown here, have a limited number of specialised cells – for example in the stipe, holdfast and blade. The degree of specialisation and complexity of cell types is significantly less than that found in plants and animals. For example they do not form specialised cells such as muscle tissue or organs such as the heart or lungs found in animals.
They may be unicellular microscopic organisms such as diatoms, amoebas and paramecia or large multicellular seaweeds such as Brown, Red or Green algae. Brown algae have a brownish colour, which is due to the presence of a photosynthetic pigment called fucoxanthin. Green algae has the photosynthetic pigment chlorophyll and Red algae has a red pigment phycoerythrin which absorbs light in the blue and green wavelengths.
Note that Blue-Green algae are prokaryotic photosynthetic bacteria not protists and are members of the Cyanobacteria phylum. They are believed to be the origin of chloroplasts.
Fungi. This kingdom includes organisms that are eukaryotic but are unable to produce their own food. They obtain food by secreting enzymes into their surroundings to break down organic matter, which they then absorb. Fungi probably evolved before land plants first appeared. Fungal cells are of two basic morphological types: yeasts (unicellular microscopic fungi) or hyphae (multicellular filamentous fungi). Yeast is used in brewing and baking to produce carbon dioxide and alcohol from sugars. Hyphae are long, slender, filamentous structures. Hyphae cells can elongate at their tips or branch to produce a complex network called a mycelium which spreads underground or on rotting trees. Hyphae tip growth, shown here, involves the production of vesicles. The contents of the vesicles are used to break down and digest the substrate on which the fungus is growing. Mushrooms are the fruiting bodies of underground mycelia but most fungi produce spores without producing mushrooms. Mycorrhiza is a type of symbiotic relationship between the mycelium and the roots of plants. In this relationship, the fungi enter the plant’s roots and the hyphae extend out into the surrounding soil, where they absorb nutrients and water and bring them back to the plant. In return, the plant provides the fungi with carbohydrates, which the fungi use for energy. Molds are another type of fungus that consists of a network of hyphae but they colonise a surface and reproduce by producing spores which give the mold a fuzzy appearance. Some fungi produce antibiotics to control bacteria and are a source of medicines but others are human or plant pathogens. Bracket fungi are the fruiting bodies of a hyphae network growing in the heart wood of rotting trees.
Plantae. This kingdom includes multicellular organisms that have the ability to produce their own food through the process of photosynthesis using chlorophyll and have cell walls made of cellulose. Plants first appeared on land 470 million years ago and then diversified into- Bryophytes (mosses), Pteridophytes (ferns), Lycophytes (Lycopods) and the Seed plants Cycads, Gymnosperms (conifers) and Angiosperms (flowering plants). Current research, based on genome analysis, suggests that the most likely ancestor of land plants was a multicellular, freshwater green algae, such as a Charophyte, which had already developed a rigid cellulose wall. The transition to land required more specialised features.The first land plants could have been Bryophytes such as low-growing mosses but they lack roots, strength and vascular tissue. Lycopods were the first plants to produce vascular systems and grew as large as trees in the Carboniferous swamps but then declined as the climate changed and are now reduced to mosses and quillworts. Fern-like trees existed in these forest but modern Ferns only evolved 70 million years ago. Giant ferns growing up to 7m exist today. The evolution of seeds removed plants dependency on water for reproduction. Cycads were the earliest seed plants to evolve, first appearing 360 Ma. They have male or female cones, like conifers, but retain motile sperm that swim inside the female cone to complete fertilisation, a trait shared with mosses, ferns and lycopods. When Conifers evolved 300Ma they were completely independent of water for reproduction and became the dominant plant form. When flowers evolved 140Ma Angiosperms were able to outcompete Gymnosperms in many but not all environments. Using insects as pollen carriers and having seeds with fully protected embryos they had a reproductive advantage over Conifers and became the dominant plant form.
Animalia. The animal kingdom includes multicellular, eukaryotic, organisms that are heterotrophic (are unable to manufacture their own food) and have the ability to move and sense their environment. Animals may have first appeared in the seas 600 million years ago but it was in what is known as the Cambrian Explosion 540 million years ago that plant and animal forms proliferated. Soon a wide range of bizarre animal forms evolved, most went extinct but some animal types proved more successful than others and species evolved within the genetically similar groups that exist today. There are estimated to be 30 to 40 animal groups (known as Phyla) in existence now of which eight are listed on the attached diagram. These include Annelida, which includes earthworms and other segmented worms, Cnidaria, which includes jellyfish, corals, and other animals with stinging cells, Platyhelminthes, which includes flatworms and Nematoda, which includes roundworms. Others are Echinodermata, which includes starfish, sea urchins, and other animals with a spiny exoskeleton, Mollusca, which includes snails, slugs, octopuses, and other animals with a soft body and a hard, protective shell, Arthropoda, which includes insects, spiders, crustaceans, and other invertebrates with jointed limbs and Chordata, which includes vertebrates such as mammals, birds, reptiles, amphibians and fish. Based on the fossil record it is thought that all these Phyla existed at the end of the Cambrian period 485 million years ago. Since then diversification and specialisation has taken place in cells, tissues and organs. Shown in the diagram are the specialised cells that make up the four main types of tissue: epithelial tissue, connective tissue, nervous tissue, and smooth muscle. The human body, is composed of more than 200 different kinds of cells.