Biological Classification

1This chart shows one idea of how humans are related to some other

non-living species

tim

eOur species

Biological Classification• Organisms are classified into

groups called taxa (singular: taxon) based on their shared characteristics

• Carl Linnaeus (1707–1778), a Swedish scientist, is considered the father of modern biological classification (AKA taxonomy)

• His “Linnaean” system is still used today

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Carl Linnaeus

Characteristics/Traits• A characteristic or trait is an observed

structure or behavior• Example: Humans have large brains for their

size (structure) and adults walk on two legs (behavior)

Problem: Which traits are the most important when assigning taxa? For example, should whales be grouped closer to fish (both swim) or humans (both breathe air)?

Convergence is when two species evolve similar traits independently due to having similar lifestyles

Phylogenetics• Phylogenetics is the evolutionary relationship

between organisms

4This sort of diagram is called a phylogenetic tree

• DNA analysis is the most important tool in phylogenetics• If all the members of a group share an exclusive common ancestor, then the group is “monophyletic”•This is also called a “clade”

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Which of these groups is a clade?

“Ontogeny Recapitulates Phylogeny”

• Before they are born, developing organisms of different species look similar

• This similarity lasts longer in species which split from each other rather recently in evolutionary history

• Before they are born, developing organisms may have traits which they do not have when born, but which the organisms that they evolved from had when they were adults 6

Human embryos have tails and pharyngeal pouches (structures similar to gills in fish) shown here with red arrows. This tells us that human ancestors probably had tails and gills

Similarities in Development

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Egg

Adult

Taxonomic Levels• There are many different sizes/levels

of taxa• These are often based on

phylogenetics, but not always• The biggest taxon is “domain” and

the smallest is “species”• Small groups are within larger groups• For example, all members of the

family “mammalia” must be members of the phylum “chordata”

Biggest group

Smallest group

9In this diagram, the length of the lines represent time/change

Taxonomy and Phylogeny• Taxonomy is based on traits that someone thinks are

the most important traits to look at, so it’s more subjective

• Phylogeny, however, is about evolutionary relationships, so it’s more objective

• No organism or species currently living is “more evolved” in general than any other. Why is this?

Bacteria• Bacteria are single-celled (each organism is

only one cell)• They don’t have organelles• Some bacteria cause human diseases, but

others help us• Bacteria can live in many places, including

inside animals, like you!

This animation shows bacteria reproducing asexually

Archaea• Archaea are like bacteria in that they are

single-celled and don’t have organelles• They can live in extreme places where, until

recently, it was thought life cannot exist• Archaea can live in hot springs, underwater

volcanoes, and sewage treatment plants• A lot is still being learned about Archaea

A new group of archaea was recently discovered in poisonous water from mines

Archaea can live in hydrothermal vents, which are parts of an underwater volcano

Eukaryota• Eukaryote cells have organelles

and are bigger and more complex than bacteria or archaea

• Eukaryota is the only domain containing multicellular organisms, however, not all eukaryotes are multicellular

• Which domain are humans in?

Some eukaryotes

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Domain Eukaryota’s 4 Kingdoms

Fungi• Single- or multicellular• Fungi digest food

outside their body and then absorb it

• They help break down dead things and recycle nutrients

• Mushrooms, molds, and yeasts are fungi

This peach is being broken down by mold over 6 days

Protists• Protists are small and can be single- or

multicellular• Some protists are like plants and make food

from sunlight, while others are more like animals and get their food from other living things

• Live in liquid water

Here is a multicellular protist

Protists: Not a Clade

• The group “Protist” is really just a category for eukaryotes that aren’t plants, animals, or fungi

• Protists are an example of how a taxonomic group can be very different than a phylogenetic group

The groups in yellow are called protists. However, they are not a clade because they do not have an exclusive common ancestor

Human Taxonomy• Domain: Eukaryota (cells have organelles)• Kingdom: Animalia (fixed body plan and eat other

organisms)• Phylum: Chordata (have a spinal cord)• Class: Mammalia (have hair, mammary glands)• Order: Primates (forward-facing eyes and nails instead of

claws)• Family: Hominidae (large, no tail)• Genus: Homo (larger brain, walk on two legs)• Species: sapiens (even larger brain, small teeth)

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Human Evolution• Humans share a common ancestor with our closest living

relatives, chimpanzees, from about 7 million years ago• Modern humans originated in Africa about 200,000 years

ago• Much is still being learned about human evolution from

fossils and DNA evidence

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