Natural selection and genetic modification Revision Notes

Ardi (Ardipithecus ramidus) - 4.4 million years ago

Ardi is one of the earliest known hominin fossils. Discovered in Ethiopia in 1994, this partial skeleton shows a mix of ape-like and human-like features. Its pelvis and foot structure suggest that Ardi could walk upright (bipedalism) on the ground, but its divergent big toe and long fingers indicate it was also adapted for climbing trees. The skull and teeth are more similar to apes than later hominins, suggesting an early stage in the development of bipedalism.

Lucy (Australopithecus afarensis) - 3.2 million years ago

Lucy is a famous fossil skeleton found in Ethiopia in 1974. At 3.2 million years old, Lucy represents a crucial step in human evolution. Key evidence from her skeleton, particularly her pelvis and leg bones, strongly indicates that she was fully bipedal, walking efficiently on two legs. While her brain size was still relatively small, comparable to that of a chimpanzee, her bipedalism marks a significant departure from primate ancestors and a clear progression towards human characteristics.

Richard Leakey's discoveries (Homo erectus) - 1.6 million years ago

Richard Leakey and his team made significant discoveries in Kenya, including 'Turkana Boy', a nearly complete skeleton of *Homo erectus* dating to about 1.6 million years ago. These fossils show a much larger brain size compared to earlier hominins like Ardi and Lucy. The skeleton's body proportions are very similar to modern humans, indicating efficient bipedalism and a reduced adaptation for climbing. The jaw and teeth are also more human-like, suggesting a diet that might have included cooked food, further supporting the evolutionary progression towards modern humans.

Significance of Fossil Evidence

Fossils provide direct evidence of evolutionary changes over geological time. By dating these fossils and comparing their anatomical features, scientists can trace the development of key human traits, such as bipedalism, increased brain size, and changes in diet and tool use. This comparative anatomy helps construct a timeline of human ancestry and shows the gradual transition from early hominins to *Homo sapiens*.

Development of Stone Tools

Stone tools show a clear progression in complexity over millions of years, reflecting human evolution. Early tools (e.g., Oldowan) were simple, often just chipped stones for cutting and scraping. Later tools (e.g., Acheulean handaxes) were more refined, often bifacial and symmetrical, indicating greater skill. More recent tools (e.g., Mousterian, Upper Paleolithic) show advanced flaking techniques, specialization, and composite designs, demonstrating increasing cognitive abilities and technological advancement in early hominins.

Dating Stone Tools from their Environment

The age of stone tools is determined by dating the geological layers in which they are found. Stratigraphy provides relative dating, where deeper layers are generally older. Absolute dating methods include radiocarbon dating for organic materials (like wood or bone) found with tools, effective up to about 50,000 years. For older sites, potassium-argon dating is used on volcanic rock layers above and below the tool-bearing strata, providing a chronological context for the tools.

What is the Pentadactyl Limb?

The pentadactyl limb is a five-digit limb structure found in many vertebrates. It is characterised by a consistent bone arrangement: one long bone (e.g., humerus), followed by two bones (e.g., radius and ulna), then a collection of smaller wrist/ankle bones (carpals/tarsals), leading to palm/foot bones (metacarpals/metatarsals), and finally the digits (phalanges).

Evidence for Evolution

The presence of the pentadactyl limb in diverse species like humans, bats, whales, and birds, despite their very different lifestyles and functions, serves as strong evidence for common ancestry. This structural similarity, known as homology, indicates that these organisms share a common evolutionary origin, with the limb structure modified over time through natural selection to suit specific environmental demands. This pattern suggests divergent evolution from a common ancestral form.

Examples of Diversification

While the underlying bone structure remains similar, the pentadactyl limb has adapted for various functions. For example, a human arm is adapted for grasping and manipulation, a bat's wing for flight, a whale's flipper for swimming, and a horse's leg for running. These adaptations demonstrate a common blueprint being modified, rather than entirely new structures evolving independently, supporting the idea of a shared ancestor.

From Five Kingdoms to Three Domains

The five kingdoms grouped organisms largely by appearance or nutrition, putting all prokaryotes into Monera. This approach missed deeper evolutionary differences that are not visible.

Role of Genetic Analysis

Modern methods, especially sequencing rRNA genes, showed profound genetic differences within prokaryotes. Instead of being one group, prokaryotes split into Bacteria and Archaea, each genetically distinct, resulting in the new three domains model.