Did Life Begin from Space?
How did life begin on Earth? That question has intrigued humanity for millennia. In this paper I will investigate only one fairly recent theory about the origins of life on Earth: That life came from space. This theory is referred to as the “panspermia” theory because it presumes that life may be seeded from space throughout many worlds and many solar systems. By implication, the panspermia theory supposes that life should be common wherever there are physical conditions that are capable of supporting life. This report addresses the possibility that life may have originated in space rather than on Earth, and considers the evidence in support of the various theories about how that may have happened, if it did. Such theories are called “panspermia” theories from the Greek for “all seeds”—the seeds of life existing everywhere in space.
One of the critical issues in any discussion of life from beyond the Earth is how we define life at all. In most scientific circles, the modern definition of life has become “life as we currently recognize it” rather than “any possible type of life imaginable.” Yet even the definition of what we declare to be recognizable is constantly updated as we discover life here on Earth that survives in extreme conditions of heat, radiation, extreme acids, unusual chemistries (sulfur-based), and even inside rocks buried deep underground. The bottom line condition for life “as we know it” seems at this point to be the presence of liquid water at least at some time in the past, if not the present (Chybra & Hand, 2005).
Panspermia Theories of the Origin of Life
Most theories of the origin of life assume that it developed from some type of organic molecule “soup” which spontaneously developed into self-replicating molecules and from there into primitive cells. The question is, where did the organic molecules come from to initiate the process. Did they spontaneously form as the Earth cooled, perhaps by a conjunction of lightning, water, and heat acting on a warm pool of chemicals? Or, possibly, did the organic molecules—even the self-replicating molecules of RNA and DNA—derive from someplace else? Did those molecules come from outer space?
The concept that life derived from space sounds very modern and trendy, but in fact, it was originally proposed in the 17th Century by both Sir Isaac Newton and Edmond Halley (of Halley’s Comet). Their theories were not quite the same, however. Halley believed that a cometary impact on Earth could have caused the Biblical flood; Newton believed that when the Earth passed through the tail of comets it could have collected water which in turn would become the source of life on Earth (Owen, 2008).
Modern life-from-space theories take a leaf from both Newton and Halley in that it is now believed that the organic chemicals found in the tails of comets could indeed have been accreted through both cometary impacts and near-misses. It is also believed that in the early Earth, massive numbers of cometary impacts could have delivered substantial amounts of water, and may have delivered enough water to the newly formed Earth to create the oceans (Owen, 2008).
Today, however, there are a number of variations on the idea that life may have come from space. The first type of theory is that life developed initially on another planet in the solar system, and then was transmitted to Earth in the form of rocks ejected from that planet which eventually fell to Earth as meteorites (i.e., a transpermia theory of “transmission” of the seeds of life from other planets). A second set of theories considers whether any life transmission to Earth might have happened only once, at the time the early Earth was forming (i.e., a panspermia theory), or whether it is ongoing today (a neopanspermia theory). Finally, an additional set of theories considers whether life might have transmitted across interstellar space (interstellar panspermia). Each of these types of panspermia theories have their proponents.
Transpermia: Life Transmitted from Other Planets in the Solar System
Perhaps one of the most intriguing of the panspermia theories is the one that claims that life was transmitted to Earth from other planets, where it had developed while Earth was still cooling. If this theory is true, the most likely candidate for a source planet would have to be a planet that formed somewhat earlier than Earth, and which was close enough for life—in the form of either organic molecules or even bacteria or other microorganisms—to survive the journey across interplanetary space to reach Earth. The strongest candidate for such a source of life is Mars.
Mars is smaller than Earth, which means it could have cooled and solidified somewhat earlier than Earth did. Thus, life could develop there and have time to be ejected and at some later time, land on Earth to seed life here. Does science have any opinions about this possibility? As it happens, there are scientists who believe that life indeed might have originated on Mars (or some other planet in the solar system).
Tepfer and Leach (2006) studied plant seeds as a model for how organisms could potentially survive in the harsh conditions of space. The transfer of biological materials in planetary ejecta has been called transpermia. The authors note that, solely considering transfers between Earth and Mars, nearly 0.01% of the ejecta from Earth ends up on Mars within a million years. While that is clearly not a huge volume in any one incident, over the course of billions of years, the accumulation of material transferred between the two planets can be substantial. In addition, of course, transfers can happen between other planets as well.
Wickramasinghe and Wickramasinghe (2008) make a similar argument for transpermia between the upper clouds of Venus and Earth. In that case, the types of microorganisms transferred would be extremophile methanogens—organisms that live in extreme methane environments.
Panspermia (One-Time) and Neopanspermia (On-going) Seeds of Life
Wainwright (2003) noted that there were actually two versions of the idea that life came from outer space. The original form, panspermia, posited that life arriving on Earth was a one-time event early in Earth’s history. A newer form, neopanspermia, has posited life continues to shower onto Earth from interstellar dust, passing comets, and so on. Wainwright claims that radiation is the greatest barrier to the survival of bacteria or microorganisms in space because there is no barrier to the harsh UV ionizing radiation. (On Earth, our atmosphere and the ozone layer protects the biological community from radiation.) However, Wainwright notes that by clumping together, bacteria can generate a protective shell against even that threat. Also, by embedding themselves inside the rocks, the bacteria have much more protection against the space environment. Efforts to isolate samples from the upper atmosphere have been controversial, according to Wainwright because it is so difficult to guarantee that any sampled microorganisms were not ejected upwards (such as via volcanic eruptions) rather than originating in space and falling downward. Wainwright also notes that if space-originating organisms are still raining down on Earth, there should be issues with DNA compatibility. Exposure to UV radiation should have mutated at least some of those organisms to forms not found on Earth naturally, and thus they would have DNA substantially (or at least moderately) different from Earth-based forms. Wainwright argues that finding Earth-identical life in the upper atmosphere really doesn’t prove or disprove neopanspermia. Only when tests can be done in situ in space can neopanspermia be confirmed or negated in Wainwright’s view.
Interstellar Panspermia (Life from Other Stars)
Wallis and Wickramasinghe (2004) considered the possibility of panspermia occurring not simply within the solar system, but also between star systems. Such interstellar panspermia is challenging because of the long-term exposure to radiation and the harsh interstellar environment. In their paper, Wallis and Wickramasinghe directly addressed the problem of how such transmission of microorganisms across star systems might happen. They presented two potential delivery mechanisms. One is transmission after planetswere fully formed. The other mechanism considered happened when the solar disk is still a solar nebula. The authors concluded that after the system formed, interstellar transmission might have been possible, but of very low volume and highly inefficient. For example, consider an asteroid impact on Earth. Some of the material is ejected into space, and a small fraction of that may achieve escape velocity from the solar system. These ejecta (from all planets, not just Earth) accumulate in the Kuiper Belt (the cloud of comets far from the sun at the outer limits of the solar system). These eventually may accumulate as part of a comet head, which may, by chance, be jostled away from the sun and out into interstellar space. Eventually, that body may be captured by another sun and thus deliver its load of organic material across interstellar distances. In the case of the transmission at the time of a solar nebula, before the planetary bodies accreted into masses. In this case, W allis and Wickramasinghe (2004) argued that the organic molecules accrete as the comets in the Kuiper Belt accrete and then are transmitted to other star systems in a similar fashion.
Ehrenfreund and Charnley (2000) noted that organic molecules are not only common within the solar system, but also exist in the interstellar medium—space between stars. In a detailed report they identified pathways for organic materials to diffuse across interstellar distances and noted that the Infrared Space Observatory and ground-based radio-telescopes both provided evidence for the presence of organic materials throughout space.
They concluded that in the life cycle of cosmic dust, organic molecules formed in the ISM [interstellar medium] are later incorporated in solar system material, and probably supplied organic material that seeded the early Earth. (Ehrenfreund and Charnley, 2000, p. 469).
In considering how life might be transmitted across interstellar distances, Napier (2004) also noted that large meteoroid or cometary impacts on Earth end up ejecting material from the Earth into space, where they are worn away by collisions with other material into smaller and smaller particles within perhaps 10,000 years. Once those particles are small enough, on the order of one micron (one-millionth of a meter) in size, the pressure from solar radiation can accelerate the particle, just as solar radiation streams a comet’s tail backward away from the sun, whether the comet itself is approaching or receding from the sun. While small, those particles are still easily large enough to protect microorganisms from the radiation effects of space. Napier estimates that some 1020 of these particles, or about 100 billion billion of these particles are ejected about every million years. These then provide the seeds for both protoplanets and also may be accelerated by the solar wind to locations in other star systems. Finally, Napier estimates that if this process is typical, it would be possible to seed an entire galaxy in only a few billion years (Napier, 2004).