Popular Science Monthly/Volume 25/May 1884/Where Did Life Begin?
By G. HILTON SCRIBNER.
THE subject of the distribution of plants and animals has for a long time engaged the attention of many able, persistent, and discriminating investigators. Much time and effort have been expended in simply observing and describing the various means by which they get about from place to place. The methods and means by which the seeds of plants are carried and deposited in new localities, the agency of insects, birds, and other animals in their distribution, no less than their own ingenious contrivances for floating with the wind and tide, and for catching on to every moving object, all have been carefully observed and faithfully chronicled.
The first important truth enforced by these observations is that all organic life on the earth is, in a generic or tribal sense at least, migratory and nomadic. The individuals may be rooted and stationary, but the tribe is traveling, constantly leaving old fields and surroundings and as constantly arriving in new ones, sometimes crowded out, sometimes starved out, and sometimes invited out, but always moving—moving on to a new environment, better suited, taking all things into consideration, to satisfy the pressing needs of, and to develop and raise in the scale of being, both the individual and the species.
A second great truth taught by examining the methods of these movements and studying the causes of this ceaseless tramp of organic life is, that certain essential elements of the environment itself are usually found to be traveling with or a little in advance of the migratory species. In other words, the rainfall and isothermal lines, the climatic and other conditions of life, are constantly and slowly changing relative to the locality, but moving in fact. It has been frequently observed that certain species, occupying some particular territory now, have at some recent time in the past been enabled by such changes to crowd out other occupants of the same territory, and in turn will be undoubtedly, by similar changes and means, crowded out themselves. All kinds of plants and animals which have remained in one locality until they have lost the means of movement, which can not or will not travel, must sooner or later first degenerate and then be exterminated. For instance, a rain-belt or an area of dew-fall veers slowly but permanently from the north to the south; an arid soil is made fertile, and a fertile soil is left arid; the grass and flowering plants in endless variety move with the dew or the rain-belt; the deer follow the grass, and the wolves follow the deer; a thousand varieties of insects follow the flowering plants, and the insectivorous birds and other animals, herbivorous and carnivorous, bring up the rear, and so on, through all the interdependencies of life, the change of a single essential condition, the movement of one variety, causes a disturbance and movement of all in the neighborhood. Thence comes all this ceaseless and migratory activity among the flora and fauna of the earth.
This condition of things would indicate the possibility at least that life upon the earth had in the main commenced in some favored area, and traveled thence far and wide over the surface of the globe, driven out by changes of environment, lessening in effect the favorable conditions of its development in the place of its beginning, and ever beckoned on by more favorable conditions in adjacent districts. As there are no plants and no animals, with the exception of man, and possibly his companion the dog, and his pest the rat, that can thrive in most latitudes where any life is possible, so it is very evident that plants and animals, as we now see them, could not have made their advent upon the earth universally or simultaneously. Every geological fact contradicts both suppositions. Besides, to allege either is to claim, first, that all parts of the earth became habitable, for some form of life, at the same time, which is scarcely possible; and, secondly, such an allegation would do away with the main question of distribution, render superfluous most means of movement, and make it sheer nonsense to talk about the time, methods, and character of the distribution of that which had from the beginning been fully distributed. It is much more probable that life made its first advent upon this globe in some favored locality, and not everywhere at once.
It would seem as axiomatic a proposition as can be made in natural science, that life would make its first appearance on that part of the earth, or on that part of any developing planet, which by climatic and and all other concurrent conditions was first prepared, if not to originate, at least to receive and maintain it. Nothing can be more certain than that it could not make its first appearance on that part, or on any of those parts, wanting these conditions.
By concurrent conditions of climate or temperature, wherever the phrase is used herein, I mean such currents of air and ocean, such evaporation and condensation of water, such disintegration of rock, such electrical and chemical changes, new combinations, phenomena, and movements as are influenced by or accompany changing climate or temperature, together with all the secondary and remote effects caused thereby. And in speaking of the first appearance of life it matters not, to my mind, whether it was a creation, a development, or a transplantation; whether it was a lichen on the rock or a monad in the sea; a single solitary primordial cell, or one molecule of plasmic matter anywhere. This inquiry is not for the causes, methods, character, or extent of first life; it is simply and only concerning its probable primus locus.
If we are so fortunate as to discover where life began on the earth, it will be safe enough to rest upon the assumption that much, if not all, of the present life on the globe is its legitimate result and outcome.
Are there, then, any data, any accepted facts touching the condition of our globe antecedent to the advent of plants and animals which would enable us to compare and contrast its past with its present condition, and which under known laws would indicate what portion of the earth's surface first became, by temperature, climate, and other concurrent conditions, habitable for life? Can any reasonable, probable, and still existing cause be discovered occurring in the very center of such first habitable portion which would have dispersed all vegetal and animal life and sent it in equal distribution through all the seas and over all the great continents as rapidly as such other portions of the earth became by temperature, climate, and other conditions ready to receive and maintain it? Is there any one locality answering to these conditions, and yet of which it may be said, in a grander and truer sense than it was said of Rome, that all roads lead to and from it; not only highways diverging to every part of the world, but with vehicles upon them; seed-wagons running constantly in the direction of the most favorable distribution and to the remotest parts of the earth? Any locality so related to the topography of the whole earth as to render such extensive movements of plants and animals from it in all conceivable directions, and to all distances, not only easy and probable, but consistent with their present distribution? Is there anything in similarity of form, anatomy, structure, size, color, food, habits, habitat, longevity, modes of propagation, terms of gestation, and capacity for inter-breeding between certain flora and fauna of the Eastern Continents and the Western, which would suggest that many species and varieties so widely separated might have come originally from the same locality and ancestry? Are plants and animals always improved, developed, and rendered prolific more by being moved one way than another? Are the prevailing bottom currents of air and ocean in the direction of such favorable movements? Are cases of extermination and degeneration the result of a counter-movement, or a failure to make such favorable movements?
Many facts and considerations exist and may be presented pointing to a solution of these questions, and fairly answering some of them.
Let us consider, in the first place, the probable condition of the earth previous to the advent of any sort of life upon its surface. A large portion of those who have formed any intelligent opinions, in the light of modern thought and investigation, upon the subject of cosmogony, believe and hold very firmly that the earth was at one time an intensely hot globe—indeed, a molten mass—and that in the lapse of time it has cooled down by radiation to its present temperature. It is not at all necessary for the purposes of the present inquiry to examine the so-called nebular theory, nor even to ask when or how this globe became so heated, nor to what extent it has now become cooled, nor need we inquire whether the earth is now but a molten mass covered with a comparatively thin crust, or has cooled and hardened to its very center. It is important, however, to have it understood at the outset that the facts and considerations here presented are addressed to those, and those only, who have reached and adopted the conclusion that this globe, at some time in the process of its formation and development, passed through a fiery ordeal, that the primary rocks are of igneous formation, and that there are many other existing conditions and obvious facts which can not well be accounted for except upon the hypothesis that the whole earth was once a molten mass.
Even after these admissions one embarrassment presents itself, happily, however, not affecting the argument, viz.:
So fully has every conceivable inference, every supposable fact and phenomenon in the development and history of the earth, been reviewed and discussed over and over again, in the light of this primitive glowing molten mass, by able and discriminating writers, that it may seem presumptuous at this late day to attempt any new deduction, or to draw any new conclusion radically important, touching this matter. But if the views here presented have been expressed before, in the relation of cause and effect, the writer has not been fortunate enough to meet with them, and it is quite safe to say that if they are correct their significance as a factor in other problems at least will not be questioned.
It is not claimed that these views have been proved to be true inductively, but there are certain facts and phenomena pointing directly to definite conclusions hereinafter stated which I am sure every one holding and believing that the earth was at one time a molten mass will find it easier and more reasonable to admit than to deny.
Regarding the earth, then, as at one time an intensely hot globe, totally destitute of organic life, one of the principal and indispensable conditions of rendering it habitable for plants and animals evidently would be the radiation into space of its excessive and destructive heat. The accomplishment of this, with the train of concurrent effects which would follow, or at least ever have followed the gradual reduction of temperature, is all that would be necessary to render the earth a suitable place for the maintenance of vegetal and animal life. At any rate this is precisely what has taken place since the commencement of the Azoic age, and is still taking place on parts of the earth's surface to-day, visible and obvious to any observer.
Our inquiry, therefore, is reduced to this question: What part or parts of the earth's surface first became sufficiently cooled by radiation to be habitable by plants and animals?
A supposed case may help us in reaching a correct answer to this question. Let us assume, then, that the earth, at the time it was a molten mass, had been and was revolving in an orbit so near the sun that the amount of heat it would have been receiving from the sun would have just equalized the amount of heat it was losing by radiation. Under these conditions it would have cooled as the sun cooled—neither faster nor slower. This helps us to understand that the heat received by the earth from the sun is, and ever has been, an offset, so far as it goes, to the heat lost from the earth by radiation. A statement of the loss of heat from the earth during any definite time may be formulated in this way: From the heat lost by the earth by radiation during a given period subtract the heat received by the earth from the sun during the same period, and the remainder will be the earth's net or actual loss of heat. Sidereal heat received by the earth being infinitesimal in comparison, is not here taken into the calculation. But, were it more considerable, it would not be important in this connection, for it falls upon all parts of the earth about equally.
It is evident, from the present condition of the earth's surface, that at the time it was a molten mass, and for a long time thereafter, it radiated heat into space much more rapidly than it received heat from the sun; but nevertheless the heat of the sun is, and always has been, offsetting the loss of heat from the earth by radiation to the full extent of the heat which the earth had been receiving from the sun during the time.
But this sun-heat, this offset to radiation, has not been received by all parts of the earth equally. The equatorial belt, or torrid zone, has always received the most per square foot, or in proportion to its area. The two intermediate or temperate zones have received the next largest amount per square foot, or in proportion to their area; while the polar or frigid zones have received the least per square foot, or in proportion to their area. If the amount of sun-heat received at the equator be rated at 1,000, then, upon the same basis, the average of sun-heat throughout the torrid zone should be rated at 975, the average sun-heat throughout the temperate zones at 757, and the average sun-heat throughout the frigid zones at 454, or less than one half that of the torrid and less than two thirds that of the temperate zones. We speak here, and shall hereafter, of the geographical zones of the earth for the sake of convenience.
The greatest amount of heat received from the sun and offsetting radiation from the earth, other things being equal, is, of course, as we have seen, at the equator, and less and less every degree north and south of this line to the poles. If, then, the frigid zones have been during all this time receiving the least heat from the sun—the least offset to their own loss of heat by radiation—does it not follow that they were the first parts of the earth sufficiently cooled to maintain vegetal and animal life? The inference seems inevitable.