Q.Explain, with a suitable illustration, how the fossil record (palaeontological evidence) is used to support the theory of evolution. Why are older rock strata generally associated with simpler, more primitive fossil forms and younger strata with forms closer to living species?

Palaeontology is the study of fossils (remains/impressions of past organisms). Fossils found in different rock layers show the sequence and gradual change of life forms over time — this is palaeontological evidence for evolution.

Palaeontological evidence refers to fossils — the preserved remains, impressions or moulds of organisms in sedimentary rocks. Studying fossils across successive geological strata reveals which forms existed at which times, transitional forms (e.g. Archaeopteryx), and the progressive change of species, strongly supporting evolution.

The distractors belong to other lines of evidence: embryonic development is embryological evidence; homologous and analogous organs are anatomical (morphological) evidence. Only fossils are palaeontological.

A standard NCERT/CBSE Class 12 Biology Evolution board question.

Radiocarbon (¹⁴C) dating, based on the decay of carbon-14 (half-life ~5730 years), is used to determine the age of relatively young fossils/organic remains — the appropriate isotope for a maximum age around a few hundred thousand years.

Different radioisotopes suit different age ranges:

• ¹⁴C (carbon-14): half-life ~5730 years; used for recent fossils and organic material — the correct choice for young fossils (the question's ~3,50,000-year range) as opposed to isotopes used for millions/billions of years.
• ²³⁸U → ²⁰⁶Pb (uranium–lead): half-life in billions of years; used for very ancient rocks/fossils, far older than the range asked.
• ³H (tritium): half-life ~12.3 years — far too short-lived for fossil dating.

Since the question asks for the isotope used for comparatively recent fossils, the answer is ¹⁴C.

A standard NCERT/CBSE Class 12 Biology Evolution question on radioactive dating of fossils.

Archaeopteryx is the most famous transitional fossil, showing a combination of reptilian and bird-like characters, linking Reptilia to Aves (birds).

Archaeopteryx (found as a fossil from the Jurassic period) is considered the classic "missing link" connecting reptiles and birds because it possesses characters of both groups simultaneously:

• Reptilian features: teeth in the jaws, a long bony tail, and claws on the wings.
• Avian (bird-like) features: feathers covering the body and wings, and a beak-like structure.

This mosaic of features supports the theory that birds evolved from reptilian (specifically, theropod dinosaur) ancestors, providing strong fossil evidence for organic evolution.