Fossils provide our best direct evidence for the evolutionary changes that have occurred throughout Earth’s history.  Fossils are preserved remains or traces of once living organisms, which span the geologic record and are made up of organisms from microscopic to macroscopic. But how do they form? For an organism to become a fossil, the once living organism must be covered in a layer of sediment, in a process known as sedimentation, before the organism begins to decompose.

Petrified fossils

If the buried organism is sufficiently starved of oxygen, the decomposition process will nearly halt. With adequate sedimentation and hindrance of decomposition, the organic material of that organism will begin to become infiltrated with minerals, in a process known as mineralization. Fossils of this nature are known as petrified fossils. Woody plants and animals are by far the most common petrified fossils. This is due to the dense nature of the wood in trees and bones in vertebrates. 

Preserved fossils

Fossils are not always petrified remains of once living organisms. In rare cases, organisms can be entombed by a sticky substance that eventually solidifies. These fossils are known as preserved fossils. The most common preserved fossils are encapsulated by sticky resin excreted by plants. The main difference between petrified and preserved fossils is that preserved fossils still contain organic matter, whereas petrified fossils are composed of minerals that infiltrated the organism.

Cast fossils

Soft-bodied organisms are much more likely to decompose prior to complete mineralization. Although this is true, we do have fossil evidence of soft-bodied organisms. While most of these organisms are not petrified, many have left behind evidence of their existence. For example, worms leave burrow holes and ants build underground nests. These cavities eventually fill with sediment, having the potential to fossilize.  Fossils such as this are known as cast fossils.

Trace fossils

Trace fossils are formed when an impression is left in rock by the organisms. Fossilized footprints are examples of trace fossils. Plant fossils are nearly always trace fossils. In certain cases, when a plant decomposes, it has a chemical reaction which alters the color of the rock, making the impression very clear. 

Comparative Anatomy of Primates

Scientists have been developing the evolutionary relationships among living organisms long before the recent advances in DNA technology, which have allowed us to construct genetic homologies. Prior to this technological advance, evolutionary scientists based their relationships on physical characteristics of organisms. Modern evolutionary science is based both on physical similarities (structural homologies) as well as genetic homologies. For living species, we can look at a variety of physical characteristics. However, for extinct species we are entirely dependent on the analysis of the fossil record to develop these relationships. In this exercise, you will discover many of the similarities and differences among skulls of humans, human ancestors, and our closest living relatives: the great apes. 

Dental Arches

Teeth of an organism tell us a great deal about how they lived. Herbivores tend to have very large molars for chewing vast quantities of vegetation. Whereas carnivores’ canines are very large and sharp for tearing meat. Humans and their relatives are generally omnivorous, and their teeth are intermediary of either strict herbivores or strict carnivores. 

In this lab, you will contrast the dental arch structure of the great apes (also known as pongids) with humans and their closest ancestors (the homininds). Specifically, you compare anatomical differences between two pongids - the gorilla (Gorilla gorilla) and the chimpanzee (Pan troglodytes) - and two hominds - Australopithecus afarensis and the modern day human (Homo sapiens). 

When comparing the dental arches of these organisms, contrast the overall shape of the dental arch. Humans, for example, are said to have an U-shaped (or parabolic) dental arch. How does that compare with Australopithecus, Gorilla and Pan?

Foramen magnum

The foramen magnum is the hole in the base of the cranium where the spinal cord enters into the cranium (or braincase). The position of the foramen magnum is a clear indicator of how the animal walked. In the animation below, you will see the foramen magnum of a human highlighted in red. If you place the eyes forward the foramen magnum of a human is perfectly vertical. This indicates that the spinal column of a human enters into the cranium at a vertical angle to the ground, which scientists use to infer that this organism is bipedal, or walks up right on two feet.  

If you compare the foramen magnum with a quadriped (or animal that walks on four legs) with a biped, a clear distinction appears. Below, you can see that the foramen magnum of a dog (Canis domesticus) is horizontal relative to the ground, whereas humans is horizontal. The metal shaft extending to the right is inserted in the center of the foramen magnum of the dog's skull.  

Below is an animated illustration of a lion walking from E. Muybridge's Descriptive Zoopraxography (1893). You can visualize where the spine enters into the lion's cranium from where the neck bends closest to the head. Lion's also have a horizontal foramen magnum. 

Chimpanzees and gorillas are considered "knuckle-walkers." While they can walk on two legs, they typically walk on fours, using their back legs in most of the locomotion (but not entirely independent of the front arms. If you, visualize the insertion of the spinal column into the foramen magnum of the cranium, it inserts at a different angle than the lion, not horizontal but also not vertical. 

Below are several examples of the insertion angles of the spinal column into the foramen magnum of pongids and hominids. From these angles, we can infer how extinct organisms walk. From your understanding of the foramen magnum, answer the following questions:

• How did Homo erectus walk?
• Was Australopithecus bipedal or quadrupedal?