(a) Describe the stages of human evolution and mention the major hominid forms giving their time of origin. (20 marks) (b) What…

(a) Stages of Human Evolution and Major Hominid Forms

Human evolution is the sequence of hominin forms in which bipedalism appeared first and brain enlargement, dental reduction and tool culture followed. The main stages, with approximate dates of origin, are:

• Miocene apes (background): Dryopithecus (about 12–9 million years ago, Mya) and Ramapithecus (about 14–8 Mya). Ramapithecus, once proposed as the first hominid from the Siwaliks, is now included in Sivapithecus, which is related to the orangutan lineage and is not a human ancestor.
• Earliest hominins (late Miocene): Sahelanthropus tchadensis (about 7 Mya, Chad), Orrorin tugenensis (about 6 Mya, Kenya) and Ardipithecus (5.8–4.4 Mya, Ethiopia; "Ardi" of A. ramidus), showing early bipedal traits combined with ape-like features.
• Australopithecines (Pliocene): A. anamensis (about 4.2–3.9 Mya), A. afarensis (about 3.9–2.9 Mya; "Lucy" and the Laetoli footprints) and A. africanus (about 3.3–2.1 Mya; the Taung child). They were habitually bipedal, with cranial capacity of about 400–550 cc, smaller canines and no consistent tool making. The robust Paranthropus species (about 2.7–1.2 Mya) were a side branch specialised for heavy chewing.
• Homo habilis (about 2.4–1.4 Mya): brain about 500–700 cc, first clear link with Oldowan stone tools.
• Homo erectus (about 1.9 Mya–110 thousand years ago): brain about 800–1,200 cc, human-like body proportions, Acheulean hand-axes, probable use of fire, and the first dispersal out of Africa into Asia (Java, Peking man).
• Homo heidelbergensis (about 700–200 thousand years ago): brain about 1,100–1,300 cc; widely regarded as the common ancestor of Neanderthals and modern humans. The Narmada hominin from Hathnora in Madhya Pradesh, India's only Middle Pleistocene fossil hominin, is usually placed with archaic Homo of this grade.
• Homo neanderthalensis (about 400–40 thousand years ago): Europe and western Asia; brain about 1,200–1,700 cc, robust cold-adapted body, Mousterian tools and burial of the dead. Denisovans, known mainly from DNA, lived in Asia at the same time, and both groups interbred with modern humans.
• Homo sapiens (about 300 thousand years ago–present): oldest fossils from Jebel Irhoud, Morocco; brain about 1,300–1,500 cc, symbolic art and language, and dispersal out of Africa after about 70–60 thousand years ago to all continents.

Throughout this sequence, the skeleton shows bipedalism (S-shaped spine, bowl-shaped pelvis, forward foramen magnum, arched foot) well before major brain expansion, while the jaws and teeth became smaller as diet and tool use changed.

(b) Hardy–Weinberg Law; Change of Gene Frequency by Mutation and Genetic Drift

The Hardy–Weinberg law states that in a large, randomly mating population with no mutation, selection, migration or genetic drift, allele and genotype frequencies remain constant from generation to generation. For two alleles A and a with frequencies p and q (p + q = 1), the genotype frequencies are p² (AA) + 2pq (Aa) + q² (aa) = 1, and equilibrium is reached after a single generation of random mating. The law is a null model: a significant departure from p², 2pq and q² (tested by χ²) shows that an assumption is violated and evolution is occurring.

Mutation changes gene frequency by converting one allele into another. If A mutates to a at rate u per generation and a back to A at rate v, the change in the frequency of A is Δp = vq − up, and an equilibrium is reached at q̂ = u/(u + v). Because mutation rates are low (about 10⁻⁵–10⁻⁶ per gene per generation), mutation alone changes frequencies very slowly, but it continually supplies the variation on which selection and drift act.

Genetic drift is the random change in allele frequency caused by sampling of gametes in a finite population. The variance of change per generation is pq/2N, so drift is strongest when the effective population size (Nₑ) is small. An allele's chance of eventual fixation equals its current frequency, and heterozygosity falls by a proportion of about 1/(2Nₑ) each generation, i.e. H′ = H(1 − 1/2Nₑ). Drift can fix or eliminate alleles regardless of their fitness. The bottleneck effect (for example the cheetah) and the founder effect (for example high frequencies of certain recessive disorders in small founder communities) are special cases.

(c) Systematics versus Taxonomy; Classical versus Molecular Taxonomy

Systematics is the scientific study of the kinds and diversity of organisms and of all relationships among them (Simpson); it includes taxonomy together with phylogeny, evolution and biogeography. Taxonomy is the theory and practice of identifying, describing, naming and classifying organisms according to rules of nomenclature. Taxonomy is thus a part of systematics.

Classical and molecular taxonomy differ as follows:

• Basis: classical taxonomy uses morphology, anatomy, embryology, cytology (karyotypes) and behaviour; molecular taxonomy uses nucleotide and amino-acid sequences and other molecular markers.
• Tools: classical work relies on museum and type specimens, dissection and microscopy, dichotomous keys, and numerical taxonomy (phenetics) with character matrices; molecular work uses PCR and DNA sequencing, DNA barcoding with the mitochondrial COI gene (about 648 bp), 16S/18S rRNA and mitochondrial DNA, microsatellites and SNPs, RFLP, protein electrophoresis, and computer tools such as BLAST, ClustalW and MEGA for alignment and phylogenetic trees (maximum likelihood, Bayesian methods), increasingly using whole genomes.
• Strengths and limits: classical taxonomy is cheap, works with fossils and is needed for naming and field identification, but can be misled by convergence and by phenotypic plasticity; molecular taxonomy detects cryptic species and resolves deep relationships, but needs good-quality DNA, can be confused by incomplete lineage sorting or hybridisation, and cannot be applied to most fossils.

Modern integrative taxonomy combines both lines of evidence.