Our Place

01 · A BRANCHING STORY

Human origins: evolution is a branching tree

Fossils, genetics, archaeology, and comparative anatomy jointly constrain the broad path of human evolution. Several hominin groups coexisted for long periods. Dates are approximate, and new evidence can revise the branches.

01
About 7-6 million years ago
Earliest hominins
The common ancestor of the human and chimpanzee lineages lived earlier; early fossils show that traits for upright walking emerged gradually.
02
About 4.4-2 million years ago
Australopiths and other early hominins
They walked upright while retaining climbing abilities, and several parallel branches developed in Africa.
03
About 2.8-1.6 million years ago
Early Homo
Stone-tool use became more consistent, while diets and brain size changed. Evolution was never a one-way ladder of progress.
04
About 1.9 million-110,000 years ago
Homo erectus and relatives
Body proportions became more like ours. Populations spread beyond Africa and persisted in some regions for long periods.
05
About 400,000-40,000 years ago
Neanderthals and other archaic humans
Several Eurasian populations adapted to different environments and exchanged a limited amount of DNA with early Homo sapiens.
06
About 300,000 years ago-present
Homo sapiens
Homo sapiens emerged in Africa. Language, symbols, and cumulative culture supported cooperation across larger groups and generations.

The phrase 'humans came from monkeys' is misleading. Humans and living apes share older common ancestors, and every living species has followed its own branch for the same amount of evolutionary time.

02 · THE HUMAN BODY

Human physiology: shared systems and reproductive differences

The body is organized into cells, tissues, organs, and organ systems. Most systems share the same basic arrangement in everyone. The main typical anatomical differences concern reproductive organs, gamete production, hormone regulation, and secondary sex traits. Human development and sex characteristics vary naturally.

Eight shared systems working together

Body systems are not isolated parts. The nervous and endocrine systems send control signals; circulation moves materials; respiration and digestion provide inputs; urinary, immune, and musculoskeletal systems maintain the internal environment and enable action.

Cells → tissues → organs → systemsHomeostasis depends on feedbackStructure varies with age and individual development

Nervous system

The brain, spinal cord, and peripheral nerves receive sensation, integrate information, and control movement and automatic functions.

Endocrine system

The hypothalamus, pituitary, thyroid, adrenal glands, and gonads use hormones to coordinate metabolism, growth, and reproduction.

Circulatory system

The heart and vessels transport oxygen, nutrients, hormones, heat, and metabolic products while maintaining tissue perfusion.

Respiratory system

Airways and alveoli provide ventilation and gas exchange. Neural and blood-chemistry signals jointly regulate breathing.

Digestive system

The mouth, stomach, intestines, liver, gallbladder, and pancreas digest food, absorb nutrients, and process metabolic products.

Urinary system

The kidneys filter blood and regulate water, electrolytes, and acid-base balance. Urine passes through the ureters, bladder, and urethra.

Immune and lymphatic systems

Bone marrow, thymus, lymph nodes, spleen, and immune cells identify threats while limiting excess inflammation and self-attack.

Musculoskeletal system

Bones, joints, skeletal muscles, tendons, and ligaments provide support, protection, movement, mineral storage, and heat.

ANATOMY IN DETAIL

Male and female reproductive anatomy

The descriptions below summarize typical adult anatomy. Reproductive and urinary organs are close together but have different functions. Hormone axes change with puberty, menstrual cycles, pregnancy, menopause, illness, and treatment.

Typical male reproductive anatomy

01Testes: Produce sperm and secrete hormones including testosterone. Their position in the scrotum helps maintain a lower temperature.
02Epididymides: Coiled structures attached behind the testes where sperm mature and are stored temporarily.
03Vasa deferentia: Muscular ducts that transport sperm from the epididymides toward the ejaculatory ducts during ejaculation.
04Seminal vesicles: Secrete fluid rich in fructose and other compounds that forms a substantial part of semen.
05Prostate: Sits below the bladder around the proximal urethra and contributes secretions to semen.
06Penis: Contains erectile tissue and serves both urination and the delivery of semen into the reproductive tract.
07Urethra: Runs from the bladder to the exterior. In males it carries urine and semen, although the two normally do not pass at the same time.

Typical female reproductive anatomy

01Ovaries: Produce oocytes and secrete hormones, including estrogens and progesterone, that participate in cycle regulation.
02Uterine tubes: Fimbriae lie near each ovary. Cilia and muscular contractions help move the oocyte toward the uterus; fertilization often occurs in a tube.
03Uterus: A hollow muscular organ whose lining changes through the cycle; it is also where an embryo implants and pregnancy develops.
04Cervix: The narrow lower portion of the uterus connecting to the vagina. Cervical mucus changes with hormonal state.
05Vagina: An elastic muscular canal connecting the cervix to the vulva, involved in menstrual flow, intercourse, and childbirth.
06Vulva: External structures including the mons pubis, labia majora and minora, clitoris, and vestibule. The clitoris has dense sensory innervation.
07Hormones and menstrual cycle: The hypothalamic-pituitary-ovarian axis regulates the follicular phase, ovulation, and luteal phase. Cycle length and symptoms vary normally.

This section provides anatomy and physiology education. It cannot determine sex, diagnose disease, or replace an examination. Persistent pain, unusual bleeding, a lump, urinary difficulty, or reproductive-health concerns require a qualified clinician.

03 · OUR HOME

Earth and the Moon: life's home and its nearest neighbor

Earth is the only world currently confirmed to support life. The Moon preserves a record of early Solar System impacts and drives tides through gravity. The Earth-Moon system also informs models of planet formation, orbits, and long-term climate.

Start with the planet beneath us

Liquid water, suitable pressure and temperature, atmospheric circulation, a magnetic field, and long-lived geology shape a habitable environment. Life in turn changes the atmosphere, soil, and oceans through photosynthesis, rock formation, and nutrient cycles.

Age of Earth: About 4.54 billion years
Mean Earth radius: About 6,371 km
Surface liquid water: Oceans cover about 71% of the surface
Mean lunar distance: About 384,400 km
Lunar orbital period: A sidereal month is about 27.3 days
Origin of the Moon: The leading model involves an early giant impact

The Moon does not make its own visible light; we see reflected sunlight. Lunar phases result from the changing Sun-Earth-Moon geometry, not Earth's shadow. The Moon enters Earth's shadow only during a lunar eclipse.

04 · WIDER HORIZONS

From the Solar System to the observable universe

Astronomical distances make more sense as orders of magnitude. Light travels about 300,000 km each second in a vacuum, yet sunlight still takes about 8 minutes 20 seconds to reach Earth and roughly 100,000 years to cross the Milky Way.

Local diagram of the Sun and the relative orbits of the eight planets

Solar System

The Sun contains about 99.86% of the Solar System's mass. Its gravity dominates eight planets, dwarf planets, asteroids, comets, and dust. Orbit sizes and body diameters are not shown to a common scale.

Earth's orbital semi-major axis: 1 AU
Neptune's mean distance: about 30 AU
The heliosphere extends far beyond the planets

Milky Way

The Milky Way is a barred spiral galaxy roughly 100,000 light-years wide, with hundreds of billions of stars plus gas, dust, and dark matter. The Sun lies near the Orion Arm, about 26,000 light-years from the center.

One solar orbit: about 230 million years
A supermassive black hole occupies the center
The Milky Way belongs to the Local Group

Observable universe

The universe is about 13.8 billion years old. Because space has expanded, the observable universe is now about 93 billion light-years across. 'Observable' describes the region whose signals can reach us, not an edge of the whole universe.

Galaxies form groups and clusters
Large-scale structure resembles a web
The total size of the universe remains unknown

FAQ

Common questions about humans, Earth, and the universe

Did modern humans evolve from monkeys alive today?

No. Humans and other living primates share older common ancestors, after which each lineage followed a different evolutionary branch.

Do males and females have completely different organ systems?

No. Everyone shares the same major organ systems. Typical differences are concentrated in reproductive anatomy, gametes, and parts of hormone regulation.

Does Earth's shadow cause the phases of the Moon?

Usually no. Phases depend on how much of the Moon's sunlit half faces us. Earth's shadow crosses the Moon only during a lunar eclipse.

Is the Solar System at the center of the Milky Way?

No. The Sun lies near the Orion Arm in the galactic disk, about 26,000 light-years from the center.

Is the observable universe the entire universe?

Not necessarily. It is only the region whose light or other signals have had time to reach us since the early universe. The whole universe may be much larger.