* One billion people in Africa in 2009

* Continent's population to hit 2 billion by 2050

* Huge investment needed to cater for growth

JOHANNESBURG, Oct 21 (Reuters) - One day this year, in all probability, the "billionth African" will have been born, a milestone that will only benefit the poorest continent if it can get its act together and unify its piecemeal markets. Nobody knows, of course, when or where in its 53 countries the child arrived to push Africa's population into ten figures.

The U.N. merely estimates that in mid-2008 there were 987 million people, and in mid-2009, 1,010 million.

Given the difficulties of obtaining accurate data from the likes of Nigeria, where provincial population figures are often hostage to the ambitions of local politicians, or any data at all from the likes of Somalia, experts are reluctant to hazard any greater degree of accuracy. There is less doubt, however, about the underlying trend -- that Africa's population is set to grow faster than in any other part of the world in the coming decades, and to double by 2050.

"Despite the fact we have these huge populations in China and India, the actual growth of the population will be much more in Africa than in Asia," said Gerhard Heilig, head of the U.N.'s Population Estimates and Projections Section.

To some, the statistics will invite comparisons to the Asian giants, and inspire hopes of a flood of investment from Africans and outsiders to meet the needs of a continent likely to be home to one in five people by the middle of this century.

By contrast, China's projected population of 1.4 billion in 40 years will be shrinking, while India will only be adding an annual 3 million to its 1.6 billion people.

To others, the numbers are stark reminders of the mammoth task Africa's leaders face in providing the food, jobs, schools, housing and healthcare that are still so sorely lacking.

UNFPA, the U.N.'s population arm, summarises by saying that sub-Saharan Africa faces "serious political, economic and social challenges" and points to the last two decades as evidence that more people does not mean more wealth.

"Twenty years of almost three percent annual population growth has outpaced economic gains, leaving Africans, on average, 22 percent poorer than they were in the mid-1970s," it says.

"UNITED STATES OF AFRICA"?

From 2003 to 2008, Africa experienced an unprecedented boom due to a mixture of debt forgiveness, free market reforms and soaring commodity prices that lifted annual output by five percent or more -- crucially outpacing population growth.

That came to a juddering halt with this year's global economic slump, but the International Monetary Fund is forecasting African growth at 4.0 percent for 2010, against 1.7 percent for 2009.

If it can sustain this, and consolidate its patchwork of small countries and 30 overlapping trade blocs into a single, huge market, Africa has a chance of unlocking the 'demographic dividend' that sucked investment into India and China, analysts say.

"If that doesn't work, the demographic dividend is off. It'll just be a lot of small, unsustainable states competing against each other, as we've seen for the last 50 years," said Patrick Smith, editor of Africa Confidential magazine.

Pan-Africanism, including even a 'United States of Africa', has been a rallying cry since the continent started to shake off its colonial shackles in the 1950s and 1960s.

The reality, however, has seldom matched the rhetoric as first the polarising framework of the Cold War and then short-term national self-interest hampered growth in cross-border trade, investment and political cooperation.

Today, intra-regional trade accounts for just 9 percent of Africa's total commerce, compared to nearly 50 percent for emerging Asia, according to U.N. trade body UNCTAD.

However, there are signs this might be changing, most notably with an agreement last year by three major blocs -- the Common Market for Eastern and Southern Africa, the East African Community and the Southern African Development Community -- to create a single free trade zone encompassing 530 million people.

Implementation will inevitably hit snags and delays, but at a practical level, everything from more cross-border bus routes to electricity lines and regional 'power pools' all point to closer regulatory and political alignment.

"It's not just a political slogan now. There are some actual actions," said UNCTAD Africa specialist Janvier Nkurunziza.

SOUTH AFRICA SETS THE TONE

Set against this new political will, however, is the sheer scale of the investment needed to address Africa's problems.

The World Bank's International Finance Corporation estimates that Africa spends only $10 billion a year on upgrading its dilapidated electricity grids -- compared to $40 billion needed to meet demand forecast to treble in the next 20 years.

Similarly, sub-Saharan Africa needs to invest $11 billion a year in farming to feed the extra mouths in 2050, the U.N.'s Food and Agriculture Organisation said this month.

And even if they secure the cash, leaders need only look at relatively wealthy South Africa, where millions of blacks still live -- and frequently riot -- in shanty towns 15 years after the end of apartheid, to realise that rolling out infrastructure on a grand scale is far from simple.