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Bones of Contention

Explore the challenges scientists face when classifying hominid fossils by using this database to classify your own collection of 10 mystery fossils. Identify your fossils by comparing their features to known hominid fossils, and defend your classifications with peers. By using known facts about fossils to define hominid species, take part in the ongoing scientific process of discovering human origins.

Launch Interactive

Do the work of scientists, using this interactive database to classify actual fossil records of human ancestors.

Background Essay

In this activity, you will explore challenges scientists face in classifying fossils by attempting to classify 10 “mystery fossils.” Examine the data associated with each fossil to decide on the genus and species to which it belongs. Interpret the facts associated with each fossil to explain how each classification helps define a stage in human evolution. Assemble your own theories about the stages of human evolution, using data to support our claims. In the process, we will see where the fossil record tells a clear story, and what gaps remain.

In the 150 years since Darwin described the origin of species, scientists have gathered extensive fossil evidence in support of his theory. This evidence helps us understand the stages through which humans have evolved from a common ancestry with apes.

Key Characteristics of Hominin Evolution
Many scientists focus on bipedalism, diet, and cranial capacity as key characteristics to understand human evolution. For example, bipedalism (i.e., walking on two legs) is used to separate the earliest human ancestors from the ancestors of early apes. In bipeds, the foramen magnum—where the spinal cord enters the skull—is located near the middle of the skull, whereas in quadrupeds it is located toward the back of the skull. Likewise, the shape and position of the pelvis (hip), femur (leg), and tibia (shinbone) distinguish bipeds from quadrupeds, as does the relative lengths of their lower limbs (legs) and upper limbs (arms) and, to some extent, phalanx (finger bone) curvature.

Skull characteristics give clues about diet that help anthropologists classify species of human ancestors. For instance, the larger crests, longer faces, and extreme post-orbital constriction of modern Apes and early hominins help support the powerful jaw muscles required to consume hard diets. Scientists think that the reduction in these features that came with softer diets may have allowed for the development of larger brains in more modern hominins. In this manner, characteristics identified in fossils of human ancestors are understood through comparative evidence with apes and modern humans.

Classification of Species
Scientists often interpret such facts by classifying similar fossils into groups with similar characteristics, thus defining genera and species that represent key stages in the evolution of human ancestors. By doing this, they give structure and meaning to fossils.

Species classification, however, can be difficult. Many gaps in the fossil record remain and new fossil discoveries challenge scientists to revise their classifications. To complicate the process further, dates of fossils reveal when these human ancestors existed but determining when they first appeared and when they go extinct or evolve into other species is not possible. For example, there is no clear date to define the transition from Homo erectus to Homo sapiens or whether H. erectus even gave rise to H. sapiens.

Therefore, while defining species is essential for understanding evolutionary trends, classifying fossils involves constant reorganizing of data as new techniques and findings emerge. Many questions about our human evolutionary tree remain unanswered. Although facts associated with each fossil may be clear and general facts of human evolution are well-established, the process of classifying fossils based on those facts is one of scientific interpretation and scientists continue to debate specifics of our evolutionary story.

Download PDF Version of this essay

For Teachers

This is a student-directed activity. Download and print the Teacher's Guide: "Bones of Contention: Classifying Hominin Fossils" (PDF) for essential background information and suggestions for ways to support the activity.

Using a Fossil Database

This activity gives students access to a database containing records of most of the major fossils of our hominid ancestors found to date. Students can sort and search the fossils by feature, read about what each feature signifies, and use search filters to create groups. Students have these primary tasks:

  1. Use a database to classify each of the 10 mystery fossils by comaring them to fossils of known hominid species. Or, if the fossil is unique, create a new group.
  2. Compare and defend their classifications with peers.
  3. Compare their findings to the species names given by scientists.

Key Vocabulary

  1. hominid: The group consisting of all modern and extinct Great Apes (that is, modern humans, chimpanzees, gorillas and orangutans plus all their immediate ancestors). In formal biological classification, hominidae are a family.
  2. hominin: The group consisting of modern humans, extinct human species and all our immediate ancestors (including members of the genera Homo, Australopithecus, Paranthropus and Ardipithecus). In formal biological classification, hominini are a tribe.
  3. classification: Biological classification, or scientific classification in biology, is a method by which biologists group and categorize organisms by biological type, such as family, tribe, genus, species.
  4. genus: In biology, a genus (plural: genera) is a taxonomic unit (a taxon) used in the classification of living and fossil organisms. In this activity, we explore several genera within the hominin family, such as: Australopithecus, Paranthropus, and Homo.
  5. species: In biology, a species is a taxonomic unit (a taxon) used in the classification of living and fossil organisms. For instance, within the genus Australopithecus, species include afarensis, africanus, and garhi. Within the genus Homo, species include habilis, erectus, and sapiens.
  6. bipedal: Walking on two legs. A defining characteristic of hominins, this refers to having the ability or inclination to walk upright.
  7. cranial: Describing the the skull, or more specifically the portion of the skull enclosing the brain. A related term: post-cranial refers to all or part of the skeleton apart from the skull.

Download the full Teacher's Guide

Student Resources

Use this worksheet to guide your investigation and to record your classifications of the mystery fossils.

Download the Student Worksheet


Our Origins: Discovering Physical Antropology, by Clark Spencer Larsen. W. W. Norton & Co, 2008 (3rd edition).

Human Origins: The Fossil Record, by Clark Spencer Larsen, Robert M. Matter, and Daniel L. Gebo. Waveland Press, 1998.

The Human Evolution Source Book, by Russell L. Ciochon and John G. Fleagle. Prentice Hall, 2005 (2nd edition).

A Photographic Atlas of Physical Anthroplogy, by Paul F. Whitehead, William K. Sacco, and Susan B. Hochgraf. Morton Publishing Company, 2005.

The Human Evolution Coloring Book, by Adrienne Zihlman, Coloring Concepts Inc. Collins Reference, 2001 (2nd edition).

Body proportions of Homo habilis reviewed, by Martin Haeusler and Henry M. McHenry. Journal of Human Evolution, 2004 (46:433-465).

Turkana Basin Institute Website.



Inquirium LLC


Creative Bottle

Science Advisor

Elizabeth Weiss, Ph.D., San Jose State University


Fossil illustrations are based on casts that were provided courtesy of:

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