Phylum

The Phylum is a critical level of biological organisation used to group organisms based on their general body plan. Interestingly, Carl Linnaeus did not use the word "Phylum" in his original 1735 system; it was added in the 19th century to help scientists organise the massive diversity of life they were discovering.

The Body Plan Rule

A Phylum like Arthropoda is considered very "stable" because every member—whether it’s a tiny spider or a giant crab—shares a clear, unique ancestor that evolved a specific body plan: jointed legs and an exoskeleton.

In contrast, a Kingdom like Protista is "unstable" because it was often a "category of convenience" for organisms that didn't fit elsewhere, rather than a true family branch.

The Shark and the Human: A Shared History

To understand how specific these categories get, let’s look at two creatures that seem worlds apart: Humans and the Great White Shark.

Despite their differences, they share the same filing space for the first three levels of lifeline.

Comparison Table: Human vs. Great White Shark 8

Level Human Great White Shark Why they group or split

Domain Eukarya Eukarya Both have complex cells with a nucleus.

Kingdom Animalia Animalia Both are multicellular, mobile, and eat food.

Phylum Chordata Chordata Both have a spinal cord or "notochord".

Class Mammalia Chondrichthyes Humans have hair/milk; Sharks have cartilage.

Order Primates Lamniformes Humans are primates; Sharks are "mackerel sharks".

Family Hominidae Lamnidae Humans are Great Apes; Sharks are White Sharks.

Genus Homo Carcharodon The specific "house" or group name.

Species sapiens carcharias The specific individual type of organism.

450 Million Years of Distance

Because you and a Great White Shark are in the same Phylum (Chordata), it means that roughly 450 million years ago, you shared a common ancestor. This ancestor was likely a small, worm-like fish with a stiff rod in its back.

• The Human Path: Your ancestors eventually evolved a bony skeleton and lungs to walk on land.

• The Shark Path: Their ancestors stayed in the water and evolved a skeleton made entirely of cartilage—the same flexible material in your nose.