Page Four 



EVOLUTION 



November, 1928 



Brains — How Come? 



By Allax Stroxg Broms 

 IV. 



ON the very first page of our family album appears a sim- 

 ple-looking fellow named Amoeba. He is only a speck of 

 living jelly, microscopically small, and formless, without head or 

 tail. He would be a regular lazy-bones if he had bones, for 

 he moves only when hunger or danger prods him. Even then 

 he just pokes around until he accidentally meets a dinner (which 

 he leisurely surrounds) or a danger (which he leisurely avoids). 

 Though nothing to brag of, he surely is of our ancestral stock. 

 Of course our family has changed a lot from this humble and 

 certainly simpleminded beginning. 



The first step in getting ahead was to get a head. As a 

 start, one of the amoebas, the soil dwelling Naegleria gruberi 

 made head and tail of himself. Usually he is just an amoeba, 

 without shape or direction in life, but presently, for a few hours, 

 he turns into a spindle-shaped thing with a head-end crowned 

 by a luxuriant crop of two sensitive swimming-hairs. This end, 

 being a head, travels ahead, bumps into things and needs and 

 develops sensitiveness. This really makes a good beginning and 

 several of the one-celled animals follow suit. Stentor, for in- 

 stance, anchors by a stalk-end and lifts up its funnel-shaped sens- 

 itive head-and-mouth-end for food. When the one-celled evolved 

 into the many-celled, the rule still held. The end that got the 

 contacts, that moved ahead and took the bumps, got the brains. 



Already, in the worms the head-end and a very primitive 

 brain are clearly developed. This brain takes the bumps and 

 passes bump messages along its nerves to the body muscles that 

 respond with saving reactions of movement. Often the head-end 

 meets food and sends food messages to the nearby muscles of 

 mouth or tentacles that then respond with feeding reactions. 

 So the head acquired the mouth, a chemical (taste-smell) sense 

 and brain parts to go with them. Then the worm turned — 

 into several sorts of animals, into insects, for example, and more 

 or less directly, into primitive fishes. Just how, we are not sure, 

 but our worm-family resemblance is still there when we are 

 very young and unborn. As part of this development, eye-spots 

 sensitive to light and shadow appeared, quite an achievement, 

 of course, but after all only a sort of touch sensitiveness to finer 

 wave vibrations pounding on the skin. Again the ahead end, 

 where sensitiveness served best, got the receiving set of eyes 

 and some more brain parts — nerves and nerve centers. By this 

 time we were really getting a head. 



Head and tail were now distinct. Movement was definitely 

 forward, with occasionally a turning bend to right or left. Body 

 shape and feet or fins were fitted to forward movement. Many 

 of the lower animals moved indifferently in any direction, but 

 now the rule was "head first," for movement was safer and 

 faster that way. Position had become important to effective 

 movement and there developed a group of position senses, among 

 them a sense of balance. Its sense organ is located in the ear 

 and consists of three semi-circular tubes lined with sensitive 

 hairs and filled with a fluid that splashes back and forth when 

 we move or tip, thus disturbing the hairs and our sense of 

 balance or movement. These tubes register movement in three 

 directions, for one is vertical from back to front, telling us when 



we tip forward or backward, the second vertical again, but 

 set sidewise to catch movements in that direction, while the 

 third is horizontal to tell us when we turn around. This is 

 the one that gives us the dizzy feeling when we have been 

 whirling rapidly around. For the enclosed liquid soon whirls 

 with us and keeps right on when we stop, making us feel that 

 we are still whirling. The business of this three-tube "laby- 

 rinth" is to keep track of our movements and balance, aided 

 somewhat by the '"feel" of our muscles and bones and by pres- 

 sures on the soles of our feet. 



This balancing organ helped the fish keep right side up. 

 Fishes are light-colored below and dark above. Viewed from 

 below, they blend with the sky ; viewed from above, they blend 

 with the dark bottom. When they turn over a bit, you catch 

 the white flash at once. The right position therefore helps the 

 fish hide. But it also helps him move fast. Shape and muscle 

 and fin are all fitted to forward movement, with swings to 

 right or left. 



But the fish has another position problem, it must head and 

 swim upstream to avoid being swept down and away by the 

 current. The eyes help, for the fish watches the banks and 

 swims to keep abreast of familiar points, but it also has a pair 

 of sense organs to register the water pressure and movement 

 on each side. They are of course up front where the current 

 presses and you and I would call them ears. At this stage, how- 

 ever, they are merely extra sensitive touch spots, somewhat new 

 in structure and the way they work. If the fish turns aside, 

 the water pressure on the upstream side increases and on the 

 the other side decreases. In response, the fish swings up-stream 

 until the pressures balance and he knows he is right with his 

 world. The ears are therefore the upstream compass of the fish. 

 The same organ serves to detect water disturbances that 

 may mean food or danger and therefore need attention. As 

 there is a pair of wave-sensitive organs, the direction of the 

 disturbance can usually be detected, the near spot feeling it 

 more than the other. When later the fishes evolved into am- 

 phibians (our frogs being of this tribe) and other land animals, 

 these two ear-spots, sensitive to water-waves, improved enough 

 to detect the more delicate air-waves we call sound. This in- 

 volved a better mechanism of the inner ear, new nerve con- 

 nections and brain centers for hearing, and the growing of an 

 outer ear, a sort of ear-trumpet to concentrate more sound waves 

 on the real working ear inside, like your open hand cupped be- 

 hind your ear to help you hear. Eventually we became acute 

 enough of hearing to distinguish slight differences in sound, an 

 important step towards speech which involves both recognizing 

 and reproducing the sounds we hear. To say a real mouthful, we 

 must first hear a real earful. Of that, more later. 



Our organs of hearing and balance are found together because 

 they began together as the position organs of the fish. Thus evo- 

 lution solves another deep mystery. 



The next number on our program will be "Babies for Better 

 Brains". Oh Baby! 



nn 



Contractile 

 yacuole 



Amoeba, — no shape 

 to brag of. 



Naegleria, — 

 with head end. 



Stentor, with 

 mouth stalk. 



The 



"brain" of an earthworm. 

 (In black) 



