human development

Development of the human body

Human development is the process of growth to maturity. The process begins with fertilisation, where an egg released from the ovary of a female is penetrated by a sperm cell from a male. The egg then lodges in the uterus, where an embryo and later fetus develop until birth. Further growth and development continues after birth, and includes both physical and psychological development, influenced by genetic, hormonal, environmental and other factors. This continues throughout life, through childhood, andadolescence into adulthood.

Before birth

Development before birth, or prenatal development (from Latin natalis, meaning ‘relating to birth’) is the process in which a zygote, and later an embryo and then a fetus develops during gestation. Prenatal development starts withfertilization and the formation of the zygote, the first stage in embryogenesiswhich continues in fetal development until birth.


Sperm fertilizing an egg

Fertilization occurs when the sperm successfully enters the ovum’s membrane. The chromosomes of the sperm combine with those of the egg to form a single cell, called a zygote, and the germinal stage of embryogenesis commences.The germinal stage refers to the time from fertilization, through the development of the early embryo, up until implantation. The germinal stage is over at about 10 days of gestation.

The zygote contains a full complement of genetic material, with all the biological characteristics of a single and unrepeatable human being, and develops into the embryo. Briefly, embryonic development have four stages: themorula stage, the blastula stage, the gastrula stage, and the neurula stage. Prior to implantation, the embryo remains in a protein shell, the zona pellucida, and undergoes a series of cell divisions, called mitosis. A week after fertilization the embryo still has not grown in size, but hatches from the zona pellucida and adheres to the lining of the mother’s uterus. This induces adecidual reaction, wherein the uterine cells proliferate and surround the embryo thus causing it to become embedded within the uterine tissue. The embryo, meanwhile, proliferates and develops both into embryonic and extra-embryonic tissue, the latter forming the fetal membranes and the placenta. In humans, the embryo is referred to as a fetus in the later stages of prenatal development. The transition from embryo to fetus is arbitrarily defined as occurring 8 weeks after fertilization. In comparison to the embryo, the fetus has more recognizable external features and a set of progressively developing internal organs. A nearly identical process occurs in other species.

Embryonic development

Human embryogenesis refers to the development and formation of the humanembryo. It is characterised by the process of cell division and cellular differentiation of the embryo that occurs during the early stages ofdevelopment. In biological terms, human development entails growth from a one-celled zygote to an adult human being. Fertilisation occurs when thesperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of prenatal development commences.Embryogenesis covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus.

The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus. The germinal stage takes around 10 days.During this stage, the zygote begins to divide, in a process called cleavage. A blastocyst is then formed and implanted in the uterus. Embryogenesis continues with the next stage ofgastrulation, when the three germ layers of the embryo form in a process calledhistogenesis, and the processes of neurulation and organogenesis follow.

In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs. The entire process of embryogenesis involves coordinated spatial and temporal changes in gene expression, cell growth and cellular differentiation. A nearly identical process occurs in other species, especially among chordates.

Fetal development

A fetus is a stage in the human development considered to begin nine weeks after fertilization. In biological terms, however, prenatal development is a continuum, with many defining feature distinguishing an embryo from a fetus. A fetus is also characterized by the presence of all the major body organs, though they will not yet be fully developed and functional and some not yet situated in their final location.

Maternal influences

The fetus and embryo develop within the uterus, an organ that sits within the pelvis of the mother. The process the mother experiences whilst carrying the fetus or embryo is referred to as pregnancy. The placenta connects the developing fetus to the uterine wall to allow nutrient uptake, thermo-regulation, waste elimination, and gas exchange via the mother’s blood supply; to fight against internal infection; and to produce hormones which support pregnancy. The placenta provides oxygen and nutrients to growing fetuses and removes waste products from the fetus’s blood. The placenta attaches to the wall of the uterus, and the fetus’s umbilical cord develops from the placenta. These organs connect the mother and the fetus. Placentas are a defining characteristic ofplacental mammals, but are also found in marsupials and some non-mammals with varying levels of development. The homology of such structures in various viviparous organisms is debatable, and in invertebrates such asArthropoda, is analogous at best.

After birth

Infancy and childhood

Childhood is the age span ranging from birth to adolescence.In developmental psychology, childhood is divided up into the developmental stages of toddlerhood (learning to walk), early childhood (play age), middle childhood (school age), and adolescence (puberty through post-puberty). Various childhood factors could affect a person’s attitude formation.

Approximate outline of development periods in child development.

The Tanner stages can be used to approximately judge a child’s age based on physical development.


Puberty is the process of physical changes through which a child‘s bodymatures into an adult body capable of sexual reproduction. It is initiated byhormonal signals from the brain to the gonads: the ovaries in a girl, the testesin a boy. In response to the signals, the gonads produce hormones that stimulate libido and the growth, function, and transformation of the brain,bones, muscle, blood, skin, hair, breasts, and sex organs. Physical growth—height and weight—accelerates in the first half of puberty and is completed when an adult body has been developed. Until the maturation of their reproductive capabilities, the pre-pubertal physical differences between boys and girls are the external sex organs.

On average, girls begin puberty around ages 10–11 and end puberty around 15–17; boys begin around ages 11–12 and end around 16–17.The major landmark of puberty for females is menarche, the onset of menstruation, which occurs on average between ages 12 and 13 for males, it is the first ejaculation, which occurs on average at age 13. In the 21st century, the average age at which children, especially girls, reach puberty is lower compared to the 19th century, when it was 15 for girls and 16 for boys.This can be due to any number of factors, including improved nutrition resulting in rapid body growth, increased weight and fat deposition,or exposure to endocrine disruptors such as xenoestrogens, which can at times be due to food consumption or other environmental factors.Puberty which starts earlier than usual is known as precocious puberty, and puberty which starts later than usual is known as delayed puberty.

Notable among the morphologic changes in size, shape, composition, and functioning of the pubertal body, is the development of secondary sex characteristics, the “filling in” of the child’s body; from girl to woman, from boy to man.


In accordance with general definitions of adulthood (such as the Collins English Dictionary), biologically, an adult is a human or other organism that has reached skeleto-muscular and neural maturity; processes which have been proven to last until the mid-twenties which refer to the development of an individual’s musculature, skeletal structure, and various parts of the brain, namely the frontal lobe, with the median age range being 25-28. In human context, the term adult additionally has meanings associated with social andlegal concepts. In contrast to a “minor“, a legal adult or a ‘legal major’ is aperson who has attained the age of majority and is therefore given full independence in decision making with full support of the law; as able to be independent, self-sufficient, and responsible. The typical age of attaining adulthood is 18, although definition may vary by legal rights and country.

Human adulthood encompasses psychological adult development. Definitions of adulthood are often inconsistent and contradictory; a person may have adult behavior but still be treated as a child if they are under the legal age of majority. Conversely, one may legally be an adult but possess none of the maturity and responsibility that may define an adult, as most countries’ age of majority falls under the age of biological maturity which initself varies from individual to individual and continues to develop until the mid-twenties, with emphasis put on the brain which is responsible for the various qualities generally linked to adult behavior such as attention, executive ability and risk assessment.

Human development(economic definition) is defined as the process of enlarging people’s freedoms and opportunities and improving their well-being. Human development is about the real freedom ordinary people have to decide who to be, what to do, and how to live.

The human development concept was developed by economist Mahbub ul Haq. At the World Bank in the 1970s, and later as minister of finance in his own country, Pakistan, Dr. Haq argued that existing measures of human progress failed to account for the true purpose of development—to improve people’s lives. In particular, he believed that the commonly used measure of Gross Domestic Product failed to adequately measure well-being. Working with Nobel Laureate Amartya Sen and other gifted economists, in 1990 Dr. Haq published the first Human Development Report, which was commissioned by the United Nations Development Programme.

Central to the human development approach is the concept of capabilities. Capabilities—what people can do and what they can become-are the equipment one has to pursue a life of value. Basic capabilities valued by virtually everyone include: good health, access to knowledge, and a decent material standard of living. Other capabilities central to a fulfilling life could include the ability to participate in the decisions that affect one’s life, to have control over one’s living environment, to enjoy freedom from violence, to have societal respect, and to relax and have fun.

Our capabilities are expanded (or constrained) by our own efforts and by the institutions and conditions of our society. People with extensive, well-developed capabilities have the tools they need to make their vision of “a good life” a reality. Those poor in capabilities are less able to chart their own course and to seize opportunities. Without basic capabilities, human potential remains unfulfilled.

The capability approach is a normative framework used for analyzing well-being, often employed to understand development problems. Although certain aspects of the approach can be linked to Aristotle and Adam Smith, it is philosopher-economist Amartya Sen and more recently, University of Chicago professor of law and ethics Martha Nussbaum, who are responsible for its development and proliferation. The core premise of the capability approach is that well-being should be defined by people’s real and actual opportunities to undertake the pursuits that they desire (often referred to as ‘capabilities to function’) – and through these freedoms, be whom they would like to be. One illustration of the difference between capabilities to function and formal freedoms is found in the area of educational opportunity. All US citizens have the formal freedom to earn a college degree. However when comparing students from low-income neighborhoods with more affluent students, low-income students’ real freedoms to attend college can be constrained by, among other things, low quality local high schools and financial considerations. Formal freedoms, in this and many cases, are necessary but not sufficient to provide true capabilities to function.The capability approach to well-being, which prioritizes the ability to actualize opportunity into ‘beings and doings’, contrasts with other theories of well-being which focus on subjective measures, such as happiness, or on material means, such as income.


The state of the nation is often expressed through GDP (Gross Domestic Product), daily stock market results, consumer spending levels, and national debt figures. But these numbers provide only a partial view of how people are faring.

The Human Development Index was developed as an alternative to simple money metrics. It is an easy-to-understand numerical measure made up of what most people believe are the very basic ingredients of human well-being: health, education, and income. The first Human Development Index was presented in 1990. It has been an annual feature of every Human Development Report since, ranking virtually every country in the world from number one (currently Iceland) to number 177 (currently Sierra Leone).

This composite index has become one of the most widely used indices of well-being around the world and has succeeded in broadening the measurement and discussion of well-being beyond the important, but nevertheless narrow, confines of income. In a number of countries, the Human Development Index is now an official government statistic; its annual publication inaugurates serious political discussion and renewed efforts, nationally and regionally, to improve lives.


The Measure of America presents a modified American Human Development Index. The American HD Index measures the same three basic dimensions as the standard HD Index, but it uses different indicators to better reflect the U.S. context and to maximize use of available data. For example, while the standard index measures access to knowledge using the average number of years that students spend in school, we have chosen instead to use educational attainment, a more demanding indicator.

While data are plentiful on the extremes of affluence and deprivation in the United States, the American Human Development Index provides a single measure of well-being for all Americans, disaggregated by state and congressional district, as well as by gender, race, and ethnicity. All data used in the index come from official U.S. government sources—the American Community Survey of the U.S. Census Bureau and the Centers for Disease Control and Prevention.

The data included in the American Human Development Index will help us understand variations among regions and groups. It is a snapshot of America today. Moreover, the index will serve as a baseline for monitoring future progress.


Most people would agree that a long and healthy life, access to knowledge, and a decent material standard of living are the basic building blocks of well-being and opportunity. They are also the building blocks of the American Human Development Index as well as the U.N. Human Development Index upon which it is modeled. These three core capabilities are universally valued around the world, and measurable, intuitively sensible, and reliable indicators exist to represent them—two critical considerations in the construction of a composite index.


The most valuable capability people possess is to be alive. Advancing human development requires, first and foremost, expanding the real opportunities people have to avoid premature death by disease or injury, to enjoy protection from arbitrary denial of life, to live in a healthy environment, to maintain a healthy lifestyle, to receive quality medical care, and to attain the highest possible standard of physical and mental health.

In the American HD Index, life expectancy at birth stands as a proxy for the capability to live a long and healthy life. Life expectancy at birth is the average number of years a baby born today is expected to live if current mortality patterns continue throughout his or her lifetime. The most commonly used gauge of population health the world over, life expectancy at birth represents one-third of the overall American HD Index.

The American Human Development Project calculates life expectancy for the 50 states, the 435 congressional districts, women and men, and major racial and ethnic groups from mortality data from the Centers for Disease Control and Prevention, National Center for Health Statistics, and population data from the CDC WONDER database.


Access to knowledge is a critical determinant of long-term well-being and is essential to individual freedom, self-determination, and self-sufficiency. Education is critical to people’s real freedom to decide what to do and who to be. Education builds confidence, confers status and dignity, and broadens the horizons of the possible—as well as allowing for the acquisition of skills and credentials. Globalization and technological change have made it extraordinarily difficult for poorly educated Americans to achieve the economic self-sufficiency, peace of mind, and self-respect enabled by a secure livelihood.

Access to knowledge is measured using two indicators: school enrollment for the population age 3 and older, and educational degree attainment for the population 25 years and older. A one-third weight is applied to the enrollment indicator and a two-thirds weight is applied to the degree attainment indicator. Both indicators are from the American Community Survey, U.S. Census Bureau.


Income is essential to meeting basic needs like food and shelter—and to moving beyond these necessities to a life of genuine choice and freedom. Income enables valuable options and alternatives, and its absence can limit life chances and restrict access to many opportunities. Income is a means to a host of critical ends, including a decent education; a safe, clean living environment; security in illness and old age; and a say in the decisions that affect one’s life. Money isn’t everything, but it’s something quite important.

A decent standard of living is measured using median personal earnings of all full- and part-time workers 16 years and older from the American Community Survey, U.S. Census Bureau.

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