Part 5: Human Evolution & Migration

Hominin Lineage

The story of human evolution stretches back over 65 million years to the earliest primates. Understanding our lineage requires tracing a complex, branching tree — not a single linear chain — through dozens of species, only one of which (us) survives today. Think of it as a dense bush rather than a ladder.

Early Primates

Primates emerged in the Paleocene epoch (~66–56 million years ago) as small, insectivorous, nocturnal mammals. Key adaptations that define our order include:

• Forward-facing eyes — stereoscopic (3D) vision for depth perception in trees
• Grasping hands and feet — opposable thumbs and big toes for gripping branches
• Large brains relative to body size — enhanced visual processing and social cognition
• Nails instead of claws — flat nails with sensitive fingertip pads

Around 25 million years ago, the Old World monkeys and apes diverged. By ~14 million years ago, early apes were diverse across Africa, Europe, and Asia. The human-chimpanzee lineage split approximately 6–7 million years ago in Africa, based on molecular clock estimates calibrated by fossil evidence.

Australopithecines

The australopithecines ("southern apes") were the first hominins to walk upright habitually. They lived in Africa from roughly 4.2 to 1.2 million years ago and represent a critical transition — bipedal but still small-brained.

Homo habilis → Homo erectus → Homo sapiens

The genus Homo emerged around 2.8 million years ago with traits that distinguish us from earlier hominins: larger brains, smaller teeth, and — critically — tool manufacture.

• Homo habilis (~2.4–1.4 mya, brain ~610 cc) — "Handy Man" associated with the Oldowan stone tool culture; simple flaked pebble tools
• Homo erectus (~1.9 mya–110 kya, brain ~900 cc) — the first hominin to leave Africa; controlled fire; made Acheulean hand axes; remarkably successful, surviving for nearly 2 million years
• Homo heidelbergensis (~700–200 kya, brain ~1200 cc) — probable ancestor of both Neanderthals and modern humans; evidence of communal hunting and wooden spears
• Homo sapiens (~300 kya–present, brain ~1400 cc) — earliest fossils from Jebel Irhoud, Morocco (315 kya); symbolic behaviour, language, cumulative culture

import numpy as np
import matplotlib.pyplot as plt

# Hominin brain size evolution over time
species = ['Sahelanthropus', 'Ardipithecus', 'A. afarensis', 'A. africanus',
           'H. habilis', 'H. erectus', 'H. heidelbergensis', 'H. sapiens']
age_mya = [7, 4.4, 3.2, 2.7, 2.0, 1.5, 0.5, 0.15]
brain_cc = [360, 350, 430, 460, 610, 900, 1200, 1400]

fig, ax = plt.subplots(figsize=(12, 6))
scatter = ax.scatter(age_mya, brain_cc, s=np.array(brain_cc)/3,
                     c=brain_cc, cmap='YlOrRd', edgecolors='black', zorder=5)
ax.plot(age_mya, brain_cc, '--', color='gray', alpha=0.5, zorder=1)

for i, sp in enumerate(species):
    ax.annotate(sp, (age_mya[i], brain_cc[i]),
                textcoords="offset points", xytext=(5, 10), fontsize=8)

ax.set_xlabel('Millions of Years Ago', fontsize=12)
ax.set_ylabel('Brain Volume (cc)', fontsize=12)
ax.set_title('Hominin Brain Size Evolution', fontsize=14, fontweight='bold')
ax.invert_xaxis()
ax.grid(alpha=0.3)
plt.colorbar(scatter, label='Brain Volume (cc)')
plt.tight_layout()
plt.show()

Fossil & Genetic Evidence

Human evolution is reconstructed from two complementary lines of evidence: the physical record (fossils, tools, archaeological sites) and the molecular record (DNA from living and ancient organisms). Together they provide a far richer picture than either alone.

Comparative Anatomy

Fossil hominins reveal a mosaic of ancestral and derived traits. Key anatomical evidence for bipedalism, diet, and cognitive evolution includes:

Ancient DNA

The ancient DNA (aDNA) revolution, pioneered by Svante Pääbo (Nobel Prize 2022), has transformed paleoanthropology. By extracting and sequencing degraded DNA from fossils, scientists can now directly test hypotheses about relationships, migrations, and admixture that were previously based on morphology alone.

Key aDNA milestones:

• 2010 — First Neanderthal genome sequenced (Vindija Cave, Croatia); revealed admixture with modern humans
• 2010 — Denisovan genome from a finger bone fragment; an entirely new hominin identified from DNA alone
• 2013 — 400,000-year-old Homo heidelbergensis mitochondrial DNA from Sima de los Huesos, Spain — oldest hominin aDNA at the time
• 2022 — Svante Pääbo awarded Nobel Prize in Physiology or Medicine for founding the field of paleogenomics

Archaeological Tools

Stone tools provide a durable record of cognitive evolution. Each major tool tradition represents a leap in planning, dexterity, and abstract thinking:

Interbreeding & Diversity

One of the most revolutionary discoveries of the genomic era is that our species did not evolve in isolation. Homo sapiens interbred with at least two other hominin species — Neanderthals and Denisovans — and potentially others we haven't yet identified. This is called admixture or introgression.

Neanderthal Introgression

Neanderthals (Homo neanderthalensis) lived in Europe and western Asia from ~400,000 to ~40,000 years ago. When modern humans migrated out of Africa ~60,000 years ago, the two species overlapped geographically for roughly 5,000–10,000 years — and interbred.

Beneficial Neanderthal DNA in modern humans:

• Immune genes — HLA class I alleles from Neanderthals boosted pathogen defence in Eurasia
• Skin/hair keratin — adaptation to colder, drier climates outside Africa
• Lipid metabolism — variants that affect fat processing, potentially advantageous in Ice Age diets
• EPAS1 (partial) — some altitude-related variants may trace to archaic admixture

Denisovans

The Denisovans are a group of archaic humans identified almost entirely from DNA. In 2010, a tiny finger bone fragment from Denisova Cave in Siberia yielded a genome that was clearly neither modern human nor Neanderthal — it was an entirely new hominin lineage, known only from a pinky bone, a jawbone from Tibet, and a few teeth.

Population Bottlenecks

Genetic evidence reveals that human populations have experienced severe bottleneck events — dramatic reductions in population size that sharply reduced genetic diversity:

• Out of Africa bottleneck (~60,000 years ago) — a small founder population (~1,000–10,000 individuals) left Africa and gave rise to all non-African populations. This is why non-African populations have significantly less genetic diversity than African populations
• Toba catastrophe hypothesis (~74,000 years ago) — the eruption of Mount Toba in Sumatra may have reduced the global human population to as few as ~10,000 breeding individuals, though this remains debated
• Serial founder effects — as humans migrated further from Africa, each new founding population carried a subset of the previous group's genetic variation, creating a gradient of decreasing diversity with distance from Africa

import numpy as np
import matplotlib.pyplot as plt

# Simulating genetic diversity loss through serial founder effects
np.random.seed(42)
regions = ['Sub-Saharan\nAfrica', 'North Africa\n/Near East', 'Europe', 'Central\nAsia', 'East\nAsia', 'Oceania', 'Americas']
diversity = [1.0]
for i in range(6):
    diversity.append(diversity[-1] * np.random.uniform(0.82, 0.92))

fig, ax = plt.subplots(figsize=(10, 5))
colors = plt.cm.YlOrRd(np.linspace(0.3, 0.9, len(regions)))
bars = ax.bar(regions, diversity, color=colors, edgecolor='black', linewidth=0.8)
ax.set_ylabel('Relative Genetic Diversity', fontsize=12)
ax.set_title('Serial Founder Effect: Decreasing Diversity with Distance from Africa',
             fontsize=13, fontweight='bold')
ax.set_ylim(0, 1.15)

for bar, val in zip(bars, diversity):
    ax.text(bar.get_x() + bar.get_width()/2, bar.get_height() + 0.02,
            f'{val:.2f}', ha='center', fontsize=9)

ax.axhline(y=1.0, color='gray', linestyle='--', alpha=0.4, label='African baseline')
ax.legend(fontsize=9)
plt.tight_layout()
plt.show()

Cultural Evolution

Humans are unique among animals in our capacity for cumulative culture — each generation builds on the innovations of the previous one, creating a ratchet effect that accelerates change far faster than biological evolution alone. This is why cultural change in the last 10,000 years has been vastly more dramatic than genetic change.

Language Emergence

Language is arguably the single most important adaptation in human evolution. It enables teaching, planning, storytelling, deception, alliance-building, and the transmission of accumulated knowledge across generations.

The timeline of language is debated, but key markers include:

• ~2 mya — H. erectus likely used proto-language (gesture, vocalisations) for tool-making instruction
• ~500 kya — Hyoid bone morphology in H. heidelbergensis supports vocal capability
• ~100 kya — Symbolic behaviour (ochre use, shell beads at Blombos Cave, South Africa) implies symbolic thinking prerequisite for language
• ~50 kya — "Great Leap Forward" / Upper Palaeolithic revolution: explosion of art, music, and complex technology — likely coinciding with fully modern language

Technology Evolution

Human technological evolution demonstrates a pattern of accelerating returns. Each innovation creates the platform for the next, and the pace of change has increased exponentially:

Gene–Culture Coevolution

Gene–culture coevolution (or dual inheritance theory) describes how cultural practices can change the selective environment and drive biological evolution. Culture is not merely a product of our genes — it feeds back and shapes them.

Exercises & Review

Downloadable Worksheet

Conclusion & Next Steps

The human story is one of adaptation, migration, and cultural innovation. From bipedal australopithecines on the African savanna to modern humans colonising every continent, our journey has been shaped by a continual interplay between genes, environment, and culture. The genomic revolution has revealed that our ancestors were far more interconnected than we imagined — interbreeding with Neanderthals, Denisovans, and potentially other archaic hominins whose DNA lives on in us today.