Meat left out too long will eventually bear maggots or mold. These days we know the maggots hatch from fly eggs and the mold grows from spores that were carried in the air, but in the past the fuzzy growth and wriggling white bodies were proof that whole organisms could spontaneously arise from rotten meat or certain other types of inanimate matter.

In the early 1860s, the French chemist Louis Pasteur proved that such "spontaneous generation" did not occur, but instead the air itself was full of bacteria, spores, and other forms of reproducing life. Across the Channel, in 1859, Charles Darwin published "On the Origin of Species," which promoted the notion that life forms were ever-changing, evolving into new species over the eons.

Pasteur's experiments and Darwin's theory led to opposing conclusions regarding the origin of life on Earth. Pasteur claimed that his work lent support to the belief that God created life. Just as life could not arise spontaneously from inanimate matter, the first life on the early Earth could not have arisen without the aid of a divine creator. Yet Darwin's theory of life evolving over time implied that the first life on Earth could have evolved naturally from inanimate matter.

Toward the end of the 19th century and the beginning of the 20th century, as scientists were learning about the genetic and biochemical complexity of the cell, confusion about the origin of life grew. One way around the problem was to say that life had never emerged, but that it had always been an inherent part of the universe.

"The universe and matter were regarded then as eternal," says Iris Fry, a historian of biology at the Israel Institute of Technology and author of "The Emergence of Life on Earth: A Historical and Scientific Overview." "Life was also claimed to be eternal. Life always existed and didn't have to arise from matter. In this way, the problem of the origin of life was explained away."

Fry points to scientists such as Hermann von Helmholtz in Germany, Lord Kelvin in England, and Svante Arrhenius in Sweden, who promoted the idea of sperms of life wandering the universe and taking root in any planet with the appropriate conditions.

This idea of the seeds of life being everywhere became known as the Panspermia hypothesis ("pan" being the Latin root meaning "all"). Helmholtz, Kelvin, and others suggested that life traveled to planets within meteorites. Arrhenius and others claimed that seeds of life, protected as spores, could be pushed toward planets by solar radiation.

Fry says that the current usage of the term "Panspermia" ignores the history of ideas on the origin of life and the specific meaning of this term.

"Certainly scientists today do not believe that the universe is eternal, they do not believe that life is eternal," says Fry. "The cosmology changed. People began to realize that the universe had a beginning and that it was expanding, so this whole idea of eternity lost ground."

Fry says that the notion of eternity was used in the past to promote a philosophy that life and matter were separate unbridgeable entities. However, most scientists today agree that life arose from non-living matter.

"Scientists who believe that life might have arrived on Earth from space by meteorites or comets do not doubt that this life emerged from matter at a certain point in time on another planet," says Fry.

The term "Transpermia" is used now by several scientists to describe the transfer of life from planet to planet. Fry prefers this usage of Transpermia to Panspermia because it avoids the confusion with the old meaning.

Some scientists who study organic molecules in space have used the term Panspermia to describe the delivery of these molecules to planets like Earth. "This is neither Panspermia nor Transpermia, because it is not the transport of life, but just the transport of what might have served as the building blocks for life," says Fry.

While many of the molecules important for life have been detected in space, and meteorites and comets possibly could contain life itself, that doesn't necessarily mean life came to Earth from space. While some experiments suggest the transfer of life from one planet to another is theoretically possible, life would have to endure quite a lot to get here.

The conditions in space are extremely hostile to Earth-based life, which tends to die when exposed to an airless vacuum and extremes of temperature and radiation. Also, Earth's atmosphere acts as a barrier to life falling in from space. Recent tests conducted by European scientists have found that microbial life can not survive the fiery conditions of atmospheric entry.

Some scientists even question whether organic material delivered by comets and meteorites was necessary for life's origin, since the early Earth may have had plenty of organic material of its own.

As for what she believes, Fry says it is an open question whether or not life came to Earth from space.

"When the Earth formed, the solar system was undergoing this accretion process, and there was exchange of material between the planets, the heavy bombardment period where asteroids hit the Earth," she notes.

"It could well have been that life started on Mars, for instance, and then reached Earth. Still, even though it's a possibility, I don't see why life couldn't have started here."