Unit 4 covers 1450 to 1750 - the period in which the Eastern and Western Hemispheres are permanently joined, and the consequences of that joining.

The voyages rested on borrowed knowledge. Learning and technology from the Classical, Islamic, and Asian worlds was gathered and applied in Europe: new tools, innovations in ship design 船舶设计, and better understanding of navigation 航海 - the caravel 卡拉维尔帆船, the astrolabe, the magnetic compass, and knowledge of wind patterns.

State-supported maritime exploration 国家支持的海上探索 then followed:

• Portuguese development of maritime technology and navigational skill led to the sea route around Africa to the Indian Ocean.
• Spanish sponsorship of Columbus and subsequent voyages opened the Atlantic to the Americas.
• English, French, and Dutch crossings of the northern Atlantic followed.

Motives combined the search for wealth, competition between states, and religious mission.

New connections between the hemispheres produced the Columbian Exchange 哥伦布大交换, and the CED spells out four consequences.

• European colonisation led to the unintentional transfer of disease 疾病的非蓄意传播 to the Americas, causing catastrophic population loss.
• American foods 美洲作物 - maize, potatoes, cassava - became staple crops in parts of Europe, Asia, and Africa.
• Afro-Eurasian fruit trees, grains, sugar, and domesticated animals 驯化动物 were brought to the Americas, reshaping its landscapes and economies.
• Populations in Afro-Eurasia benefited nutritionally 营养状况改善 from a more diverse diet, contributing to population growth.

The asymmetry is the examinable point: the same exchange raised population in Afro-Eurasia and destroyed it in the Americas.

Europeans established trading posts 商站 in Africa and Asia, which proved profitable, and pursued colonisation in the Americas driven by political, religious, and economic rivalries 对抗.

Where existing states were strong, Europeans traded rather than conquered: despite disruption caused by Portuguese arrival, existing trade networks in the Indian Ocean continued 印度洋既有贸易网络得以延续, and Asian states largely dictated the terms.

In the Americas the pattern was different. Colonial economies depended on agriculture and on coerced labour 强制劳动:

• Enslavement in Africa continued in its traditional forms, including incorporation into households.
• The growth of the plantation economy 种植园经济 increased demand for enslaved labour, and the Atlantic slave trade expanded enormously to supply it.
• Systems such as the encomienda and mita 米塔制 compelled Indigenous labour.

Mercantilist 重商主义 policies were used by European rulers to expand and control their economies: colonies existed to supply raw materials and buy finished goods, and trade was directed through the mother country.

The Atlantic trading system 大西洋贸易体系 moved goods, wealth, and people between Europe, Africa, and the Americas - the pattern usually taught as triangular trade, though real routes were more complex.

Economic disputes led to rivalries and conflict between states, and commercial competition became a standing cause of European war.

Colonial societies developed new social hierarchies 社会等级, defined by ancestry and by place of birth - the Spanish casta system being the most formally codified.

Rulers everywhere faced resistance. Enslaved people resisted through rebellion 起义 and maroon communities 逃奴社群; Indigenous peoples resisted colonial demands; peasants and religious groups challenged state power across Eurasia.

The unit's reasoning skill is continuity and change. The strongest answers hold both: connection between hemispheres was genuinely new, while the dominance of agriculture, coerced labour, and land empires continued.

Worked example (a real AP exam question). AP World short-answer questions in this unit ask, for example, to "Explain one effect of the Columbian Exchange on the Americas" and "…on Afro-Eurasia." A full-mark pair states the effect and the mechanism for each: "In the Americas, transferred diseases such as smallpox caused a demographic collapse, which created the labour shortage that European colonisers met by expanding the Atlantic slave trade. In Afro-Eurasia, American crops such as the potato and maize improved nutrition and contributed to population growth." Notice the shape: effect questions want the consequence traced, and the two halves must not be the same answer twice.

• Explain the voyages with borrowed technology plus state support, not with individual explorers.
• Keep the Columbian Exchange two-directional and asymmetrical: population fell in the Americas and rose in Afro-Eurasia.
• Where Asian states were strong, Europeans traded; where disease had shattered societies, they conquered. That contrast answers many questions.
• Mercantilism is the policy, the Atlantic system is the flow, and coerced labour is what made the plantation economy work.
• Always include resistance - rebellion, maroon communities, Indigenous refusal - since it is examinable and often omitted.
• On effect questions, trace the consequence. A named change with no mechanism is only half the mark.