- 5/18/2025
The development of animal embryos reflects the evolution of species.
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00:00Tonight on NOVA, the television event of the season from the photographer who
00:06first captured the miracle of life, a stunning new special airing over three
00:12nights, Odyssey of Life. The daring vision of Leonard Nielsen opens our minds,
00:20changes the way we see, taking us on a grand journey into the tiniest of
00:26miracles. In part one, Nielsen's pioneering microphotography goes inside
00:34the human body. Join us as sperm meets egg and sets off on the ultimate journey.
00:44NOVA is funded by Merck and by Prudential, the corporation for public
01:05broadcasting and viewers like you.
01:14It is the most familiar of all miracles, life from the inside, a human embryo
01:37growing within the womb. Each of us has made this same journey, beginning with an
01:50egg cell. Twenty-three chromosomes lie within, half of our genetic inheritance.
02:07The egg cell travels through the fallopian tube.
02:13Millions of cilia hairs move it along.
02:22There's a frantic rush of sperm.
02:26And then, egg and sperm perform the dance of life.
02:51Each sperm carries its own half set of chromosomes, but only one sperm out of
03:00500 million is allowed through the egg membrane.
03:10Then, the barrier closes.
03:15The fertilized egg now has two nuclei seen in the center. When the nuclei fuse,
03:28they bring together the chromosomes of both parents, completing the set. A day
03:37later, cell division begins. Within a few hours, there are four cells, which become
03:53eight cells, which divide in turn again and again.
04:03After six days, the ovum has developed into a blastocyst.
04:14Each cell is slowly acquiring its own specialized function.
04:24When the blastocyst enters the uterus, it floats delicately down to the surface.
04:31It is here, along the uterine lining, that the next stage of growth begins.
04:46When the blastocyst has found a suitable place to settle, it must protect itself
04:51from the immune system of its own mother.
04:57It secretes an enzyme, seen here as blue crystals, a signal to the mother that the
05:06blastocyst is no intruder. Instead, it's something to be protected.
05:15Here, cell division continues, a process that doesn't end until we die.
05:27At three weeks, the embryo is just one-tenth of an inch long.
05:36But by five weeks, it has quadrupled in length. The head is bent down to the
05:43right above the placenta.
05:48At seven weeks, the embryo is about an inch long. The limb buds and the
05:57umbilical cord have taken shape.
06:07At 12 weeks, the hands and feet have already formed.
06:18At 16 weeks, the fetus is nearly six inches long, and the ears and closed
06:25eyelids can be easily seen.
06:32By 19 weeks, all its features are clearly defined.
06:39Although we pride ourselves on our uniqueness, can we even tell which one of
06:51these embryos is human? It takes weeks of growth before the answer is obvious.
07:08As we grow, we reveal a remarkable link with other species, a link that not only
07:28explains our individual development, but also the development of life itself.
07:39Consider this six-week-old human embryo.
07:48Now compare it with this embryo at a similar age. What looks like a human hand
07:55is actually the front foot of a pig.
08:00And this hand-like limb is really the nascent wing of a chicken.
08:15Our hands, their feet, and wings sharing a common ancestry that binds together all
08:22animals with backbones.
08:29Evolution has taken a simple blueprint and, over eons, built one variation after
08:57another, including ourselves.
09:15Evolution's principal mechanism is natural selection. Sudden genetic changes
09:24called mutations sometimes lead to new features in different groups of
09:28individuals. Animals that become better adapted to their environment because of
09:35these features have a better chance of survival.
09:48These better adapted individuals then pass on to their offspring the features
09:54that gave them an advantage in the first place until, eventually, the entire
10:00species is altered or a new species is created.
10:12From a bird to a panther, evolution has worked its magic.
10:24Evolution began nearly four billion years ago when meteors still bombarded a
10:42stark and tortured Earth.
10:46Somehow, this small, hostile planet came to provide the only known home for life
10:55in all the universe. Although we don't know how life began, liquids, gases, and
11:07minerals must have supplied the building blocks.
11:19Then, the first self-replicating molecules were formed, most likely in the
11:25primordial sea.
11:28Over hundreds of millions of years, these molecules reproduced themselves,
11:41adapting to their environment and growing more complex.
11:47They formed the first template of life from which everything else evolved.
11:59By three and a half billion years ago, more advanced one-celled organisms had
12:05developed. These blue-green algae are still with us today.
12:12They joined together in colonies, forming microscopic strings drifting
12:20with the current. For the next few billion years, one-celled organisms were
12:30the only life forms on Earth.
12:34They grew more diverse and even invented a new means of reproduction, which we
12:40call sex.
12:49They also produced oxygen, making possible a myriad of new organisms as
12:57the Earth's atmosphere slowly changed.
13:02A more sophisticated cell structure was at the heart of this revolution.
13:17Primitive organisms may have joined together to create this new type of cell.
13:23The result was a cell with a nucleus and other specialized organs called
13:33organelles. These gave cells the crucial ability to take on different shapes and
13:43functions. One of the most important organelles may have been the ancestors
13:52of these bacteria. Joining with the cell, they may have become mitochondria, which
13:59produce the cell's energy. Here, the mitochondria are red, almond-shaped
14:06organs.
14:13250,000 of them placed end-to-end would stretch only an inch, but their tiny
14:20size belies their importance. Inside are structures resembling the parts of a car
14:28battery with a similar function.
14:34Miniature power plants traveling through the cell. Another type of bacteria may
14:44have given organisms the ability to move under their own power.
14:50The ancestors of the spirochetes may have evolved into cilia hairs. Cilia
15:03encircle an organism like tiny legs.
15:08They allow it to move about and catch its food.
15:16They even help it perform curious rhythmic exercises.
15:47The next great evolutionary leap occurred less than a billion years ago
15:59when multi-celled organisms appeared in the ocean.
16:05Single-celled organisms living together in colonies may have been the first to
16:19make the transition.
16:21It was the lighting of a slow fuse that would one day cause an explosion of life.
16:39A dazzling array of new species appeared about five to six hundred million years
16:50ago.
16:59All of today's animals are descended from the sea creatures of this era.
17:08The sea is our ancestral home.
17:28But not every species survived from that time. Far from it. Here are some of the
17:36losers in the struggle we call evolution.
17:41Now mere fossilized remains.
17:51Those species that could not compete successfully or adapt to changing
17:56environments or had the bad luck to be in the wrong place at the wrong time
18:01were doomed to extinction.
18:07The next great evolutionary change occurred about 400 million years ago.
18:27Ancient relatives of this lungfish developed the ability to breathe air and
18:39crawled onto the land.
18:44Today's reptiles evolved from those first land vertebrates who learned how
18:59to survive out of the water. But other species would later reverse the process.
19:13A land predator resembling a dog began an extraordinary return to the sea 50
19:20million years ago.
19:23The result was the dolphin and the whale.
19:46This is the skull of that land-based predator and this is how we think its
19:54shape changed during its adaptation to aquatic life.
19:59The nose hole moved up along the head and the teeth grew smaller to accommodate a
20:05diet of fish.
20:12So the sea got something back for the life it lost.
20:16Mammals breathing air and nourishing their young with milk.
20:23The sea has kept its hold even on the mammals that stayed on land.
20:37We humans spend our first nine months in the salty water of the amniotic sac.
20:47We grow in a vestige of the primordial ocean which the first land vertebrates
20:52brought with them.
21:09The same is true of this chicken embryo inside the egg. The hard shell protecting
21:16it was an evolutionary adaptation developed during the transition from
21:21water to land.
21:32Everywhere we look in the embryonic world we find links to a common past
21:37beneath the sea.
21:41We are still linked to evolutionary changes that occurred underwater
21:52hundreds of millions of years ago.
21:58Changes that seem to have nothing to do with us like the development of gills
22:02in fish. Placed behind the head the first gills consisted of a number of
22:14arches through which blood flowed. Oxygen in the water could then pass through the
22:20arches membranes into the bloodstream. In time some of the gill arches were
22:27adapted to serve other uses. The need to digest food more efficiently or become a
22:40better hunter led to the creation of a more highly developed mouth. A mouth that
22:49could also serve as a formidable weapon.
23:03The first sign of gills appears in fish embryos at a very early stage. Here the
23:14gill arches can be seen on the left.
23:20But before the gills mature the fish embryo receives its oxygen from tiny
23:28arteries in the membrane of the yolk sac which functions like a human placenta.
23:35And even more remarkably we have the precursor to gills in an early stage of
23:53our development. The arches visible below the head look a bit like a mouth.
24:05They appear in the fourth week and for a few days develop in the same way as
24:16gills. But after two weeks or so they veer off in another direction entirely.
24:26Instead of becoming respiratory organs they develop into the lower jaw, the
24:39larynx, and the middle ear.
24:51The same traces of aquatic life can be found in other creatures that have left
24:55the sea far behind.
25:04This chick sways in its own private ocean.
25:14Like the human embryo the chick also has the precursor to gills. They're visible
25:24here as small bubbles on the throat. But a few days later they're gone.
25:45Life in the primordial ocean continues to echo through the eons.
25:54Even our spine may trace its origins to an animal without one, the ancient
26:01sea squirt.
26:07There's a curious resemblance between its eggs and ours at the moment of
26:12fertilization.
26:15There's also a resemblance between sea squirt larvae and the small vertebrate.
26:29Although the adult sea squirt has no spine, its larvae has a spine-like cord
26:45in its tail.
26:52As the larva grows it loses the tail then develops into the adult sea squirt.
26:59But what if a mutation kept a sea squirt in the larval stage all its life, thus
27:15keeping the tail intact? In that case this minute spine-like structure could
27:25have been passed on to future generations.
27:36The next evolutionary step was probably the inch-long amphioxus, which may have
27:43developed the precursor to the modern spine.
27:47You can see it here as a narrow yellow cord that runs along its back a bit to
27:58the right.
28:03From this unassuming beginning came the 60-foot long backbone of the mighty blue
28:09whale.
28:15At the backbone of the even mightier dinosaur.
28:28Our spine is a bit more modest. This is how it looks in the uterus at four weeks.
28:39The spinal segments are already shaped but still microscopic.
28:51At five weeks there's a cleavage in the back running down the center.
28:58But by six weeks the cleavage has closed and the spine is embedded inside the
29:10body.
29:20In this chicken embryo we can still see segments of the early backbone which will
29:26develop into the spine and muscles.
29:30But the end of one spine is not always the end of another.
29:56This is the tail of a pig embryo after just three weeks developing along with
30:04the spine.
30:08And here's the tail two weeks later.
30:16This chick embryo also shows signs of a tail in its early development even
30:21though the tail disappears a few days later.
30:30But what about us humans? This four week old human embryo has the beginning of a
30:39tail to the right.
30:43Like the gill arches of our aquatic ancestors vestiges of the tail appear in
30:50all of us.
30:55But by the sixth week the tail is only a small bud at the end of the spine. And by
31:06seven weeks all that's left is a little bulge below the legs.
31:11All animals without tails remain linked with those that have them.
31:28No matter how different our final form we are all bound together by a common
31:34past on the land and in the water.
31:42Even this tiny fish embryo can reveal secrets about our evolution. The
32:02development of arms and legs from fins.
32:12The process began eons ago perhaps with the ancestors of this living fossil the
32:19coelacanth.
32:26Its advanced fins may have provided the template for the development of limbs in
32:32the first land-based animals.
32:36The coelacanth has changed little over millions of years. This fossilized
32:50relative swam the oceans about 400 million years ago.
32:55By examining the bone structure of its fins we can find clues to the evolution
33:10of legs and claws. With the help of computer animation we can make the fish
33:19come alive again.
33:25We can see how the different bones in the front fins may have been modified
33:35over time until they developed into a new kind of limb useful for life on land.
33:46The animal that bore the fruits of this evolutionary change was an amphibian. The
33:55transition took place about 350 million years ago.
34:11This is the world in which the creature lived.
34:16In time the limbs of these early amphibians developed further into longer
34:29legs with toes and claws. Land-based animals, the reptiles and mammals
34:35eventually followed. Reptiles were especially successful adapting to their
34:42environment. They gained more powerful legs for walking and running.
34:53The most renowned of all were the dinosaurs who dominated the earth for
34:58more than 200 million years. Their limbs have never been equaled in size.
35:0765 million years after the last dinosaur died this same evolutionary blueprint
35:17still serves animals today.
35:22No matter where we are on the river of time we will always find a literal
35:38embodiment of the distant past.
35:42Evolution reveals itself both in the fins of fish and the hands and feet of
35:56fishermen.
36:13The buds which form our limbs can already be seen in this five-week-old
36:18embryo.
36:22This is the arm bud, as yet nearly identical to the leg bud to the right.
36:31And this is the embryo ten days later. The buds are now paddle-shaped and
36:42increasingly different from each other.
36:49But even at this stage it's still hard to tell a human arm from this front leg
36:55of a pig.
37:02And the similarity between the limb buds of this bird embryo and the human embryo
37:07is equally extraordinary. The left bud looks more like a human arm than the
37:13wing it will one day be.
37:20The development of wings may have begun with this small swift dinosaur called
37:26Comsognathus. It took millions of years but eventually the construction of its
37:36forelimb changed shape. The result was a wing with a light skeleton covered by
37:43feathers.
37:46And this is where it led to the first known bird called Archaeopteryx.
38:02Long after the dinosaurs disappeared, their relatives remained to fill the
38:08skies.
38:16We can still see traces of the dinosaurs in birds.
38:33Even a bird embryo shows us how evolution could turn a limb into a wing.
38:40Here, the wing still resembles an arm.
38:46As it develops further, it even looks like a human hand.
38:54From this point on, the wing will grow until it's fully formed, diverging more
38:59and more from the hand.
39:03But before it does, the resemblance remains uncanny.
39:11As for us, by the sixth week of life, our hands are well on their way to their
39:18final shape. Our genetic program, born of evolution, is guiding their development.
39:29To create a hand, our genes must tell certain cells to grow and others to stop
39:36at each and every stage.
39:43When the cells between the fingers are programmed to die, the shape of the hand
39:48is revealed.
39:58This embryo is in its seventh week.
40:07By the 15th week, even the fingernails are clearly visible.
40:29There are many variations, but always the same basic process.
40:37In a bat's wing, the cells are programmed to grow until a membrane of skin forms
40:44between the fingers.
41:00And so each and every species follows its own evolutionary path from a common
41:05beginning until it acquires more and more of its own special characteristics.
41:28The chicken embryo in its journey to birth.
41:35The pig embryo, from a human-like appearance to its ultimate form.
41:57The fish embryo, growing along an evolutionary path far from ours.
42:05And finally, the human embryo.
42:35This is the odyssey of life as it continues in the egg or the womb over
42:44weeks or months.
42:47This is the odyssey of life as it reflects the course of evolution in days
43:07instead of millennia.
43:17This, too, is the odyssey of life as it takes us back toward the beginning of
43:32the path.
43:48And this is our odyssey. At four weeks, a cleavage holds our first nerve cells.
44:07At five weeks, gill arches form structures that will shape the face.
44:18At six weeks, the eyes appear and the limb buds look like paddles.
44:23They grow into the center of the body.
44:29At eight weeks, the head is rounder. The eyes have moved closer and the fingers
44:34have appeared.
44:38At 11 weeks, the growing brain helps to shape the head.
44:47The eyelids have formed.
44:57At 18 weeks, the child-to-be has begun to appear.
45:08The bird, the pig, the fish, the human, all ending up in very different places.
45:27In fact, by 18 weeks, the human fetus looks like no other group of animals
45:33except one.
45:37And therein lies the final proof of our evolutionary past.
45:51You're looking at a chimpanzee fetus in its mother's womb.
45:58The chimp and gorilla are more closely related to us than to any other species
46:06on earth. They resemble us more than they do the orangutan.
46:12Chimps and humans share more than 98% of the same genes.
46:28This is an ape fetus in the 11th week.
46:38And this is a human at the same stage.
46:42During the development of both, very little distinguishes one from the other.
46:49Here is the ape fetus.
46:53And here is the human.
46:59The hands of the ape fetus.
47:11The hands of the human.
47:18The hands of the human.
47:23Only an expert can tell them apart.
47:38Here are the ape's feet at seven weeks.
47:43And here are the human's.
47:51This is the ape fetus a few weeks later.
47:57And this is the human.
48:00In some ways, the ape fetus bears a closer resemblance to a fully grown human
48:06than to a fully grown ape.
48:13Are these the hands of a human fetus or an ape?
48:18If you guessed human, you're wrong, but not alone.
48:25In evolutionary terms, it was only yesterday that we both belonged to the same species,
48:31just a few million years ago in Africa.
48:43Then we became the upright walking animals that would one day spread across the world.
49:01How did humans diverge from the early apes?
49:08How did humans diverge from the early apes?
49:15There are many theories, but one suggests that we owe our existence
49:20to an ape that remained in the juvenile stage of development.
49:27Consider the fetal development of both species.
49:33At 18 weeks, the ape's foot diverges from the human's.
49:39It comes to resemble a hand more than a foot.
49:47After birth, the ape's foot has become an impressive tool for gripping,
49:53unlike ours.
50:02Another curious fact is the ape's appearance.
50:08A juvenile can look more like a human than its own mother,
50:14but when it matures, it leaves those childish features behind.
50:21Even the growth of our brains may be linked to the differences
50:25between the two species' embryonic development.
50:33In the end, our brain becomes three times larger than the brain of an adult chimpanzee.
50:40But in the womb, our brain and head remain in a more immature state,
50:46so that after birth, they can keep growing.
51:00The chimp's brain is more fully developed at birth,
51:04and the growth is much more limited.
51:09Sometimes, it seems, immaturity pays off.
51:17And so the odyssey of life reflects the path of evolution itself.
51:30And the journey of each of us reflects the journey of our species.
51:47This is how we all begin our lives.
52:02This is how we embody the odyssey of life.
52:16A new member of the human family has just arrived.
52:47Come on, lift up your feet.
52:51Whatever the individual differences between us,
52:56we are but one species that has followed the same basic path through time.
53:04We are all the children of evolution.
53:11It's a long journey from the fertilization of the egg nine months before our birth.
53:19But it's only a fraction of the ultimate journey of evolution.
53:25Science is demonstrating that we are but one small part of a much greater odyssey,
53:33and that our part is inextricably linked to all of life.
53:53Take a closer look at the technology that's given us a closer look at ourselves.
53:58Zoom in to NOVA online at pbs.org.
54:14To order this show for $19.95 plus shipping and handling, call 1-800-255-9424.
54:23And to learn more about how science can solve the mysteries of our world, ask about our many other NOVA videos.
54:35Tomorrow night on NOVA, bloody battles, sinister traps, a throbbing heart, a loving kiss.
54:43See the world as you've never seen it before.
54:47Odyssey of Life Part II, The Unknown World.
55:01NOVA is a production of WGBH Boston.
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