MS-LS4-2: Body Structures as Evidence of Relationships
Also called NGSS MS-LS4-2 · Topic: Natural Selection and Adaptations · Life Science
The Official Wording
Apply scientific ideas to construct an explanation for the anatomical similarities and differences among modern organisms and between modern and fossil organisms to infer evolutionary relationships.
Clarification statement: Emphasis is on explanations of the evolutionary relationships among organisms in terms of similarity or differences of the gross appearance of anatomical structures.
NGSS Lead States, 2013 · Read it on nextgenscience.org · checked October 15, 2026
What MS-LS4-2 Means in Plain English
Middle schoolers compare the body structures of living animals, and of animals known only from fossils, to explain how closely they are related. Matching bones in a whale flipper, a bat wing and a human arm point to a shared ancestor, even though the limbs now do different jobs.
The comparison stays with what you can see: the shape, number and arrangement of bones and other large structures, not DNA or cell chemistry. It builds on the fossil record (MS-LS4-1) and sets up embryo comparisons (MS-LS4-3) and high school evidence for common ancestry (HS-LS4-1).
MS-LS4-2 I Can Statements
- I can find matching bones in the limbs of different animals.
- I can explain why similar structures suggest a shared ancestor.
- I can compare a fossil animal's body with living animals' bodies.
MS-LS4-2 Worked Example
A bone-matching activity a class can run with printed limb diagrams and four colored pencils.
Do a whale flipper, a bat wing and a human arm share the same bones?
Set It Up: Printed outline diagrams of a human arm, a whale flipper, a bat wing and a fossil Tiktaalik fin, plus four colored pencils.
- On the human arm, color the single upper bone red, the two forearm bones blue, the wrist bones green and the finger bones yellow.1 upper bone, 2 forearm bones, wrist, fingers
- Color the bones in the same positions on the whale flipper and the bat wing.Same red, blue, green, yellow order in all three
- Note how each limb's bones differ in length and thickness.Bat fingers very long, whale bones short and flat
- Color the fossil fin and compare it with the other three.Tiktaalik: one upper bone, two lower bones, small wrist bones
- Record which animals share the whole pattern.All four share the one-two-many bone plan
All four limbs have one upper bone, two lower bones and a set of small bones in the same order, though their shapes differ.
Why it works: Structures built on the same bone plan were inherited from a common ancestor, then reshaped over many generations for swimming, flying or grasping.
Common Mistakes with MS-LS4-2
Matching by job instead of by bones
“A bat wing and a butterfly wing are related because both fly.”
Try this: Compare the parts inside, not the job. Bat and butterfly wings are built from entirely different materials, so they do not show close kinship.
Thinking one modern animal came from another
“Humans evolved from whales because our arms match.”
Try this: Matching bones mean both animals share an ancestor that lived long ago, not that one living animal turned into the other.
Expecting related limbs to look alike
“A flipper looks nothing like an arm, so they can't be related.”
Try this: Look past the outside shape to the order and number of bones. Shapes change with use while the underlying plan stays recognizable.
Where MS-LS4-2 Fits
Practice MS-LS4-2
Quick Check
- A whale flipper and a human arm both have one upper bone and two lower bones. What does that suggest?
- A shark fin and a dolphin flipper look alike. Does that prove they are closely related?
- Why are fossils useful for this standard?