Fossils:

Definition: The preserved remains or traces of organisms (plants, animals, microorganisms) that lived in the geological past. They are typically found in sedimentary rocks. (NCERT Pg 157)
How Fossils are Formed: When an organism dies, its body parts may be buried in sediment, preventing decomposition. Over millions of years, the organic matter is replaced by minerals, or impressions are left in the rock.
Evidence for Evolution:
- Provide a direct record of past life forms, showing how organisms have changed over time.
- Help in tracing evolutionary pathways and identifying common ancestors (e.g., Archaeopteryx, a fossil bird with reptilian features).
- Show extinction events and the appearance of new life forms.
Dating of Fossils:
- Relative Dating: The simplest method. Fossils found in deeper geological strata are generally older than those found in shallower strata. It provides a sequence but not an absolute age.
- Absolute Dating (Radioactive Dating): More precise. Involves measuring the decay of radioactive isotopes (e.g., Carbon-14, Uranium-238) present in the rocks surrounding the fossil or in the fossil itself. The half-life of these isotopes allows for calculation of the fossil's age in years.

Evolutionary Path: Human evolution is a complex process involving various species of hominids, tracing back to a common ancestor shared with great apes. (NCERT Pg 159)
Tools for Tracing Human Evolution:
- Excavating Fossils: Discovering skeletal remains provides direct evidence.
- Carbon Dating: Determining the age of these fossils.
- DNA Sequencing: Comparing the DNA of different human populations and other primates to establish evolutionary relationships (molecular phylogeny).
Key Insights:
- All humans, regardless of geographical origin, belong to a single species, Homo sapiens.
- Apparent "racial" differences are superficial variations (e.g., skin color, hair texture) that have evolved over time due to adaptations to local environmental conditions and genetic drift, not distinct biological races.
- There is no "ladder" of evolution where some species are "more evolved" than others. Evolution is about adaptation to specific environments and the generation of diversity.
- The earliest members of the human species originated in Africa, and then migrated across the globe.

A: Mendel chose pea plants for his experiments due to several advantages:

They have a short life cycle, allowing for multiple generations to be studied quickly.
They produce a large number of seeds (offspring), which facilitated statistical analysis of results.
They exhibit several distinct and contrasting characters (e.g., tall/dwarf, round/wrinkled seeds) that are easy to observe.
They are normally self-pollinating, making it easy to obtain pure lines, but can also be cross-pollinated artificially.