The Birth of Homo Sapiens

人類の進化というのはテレビでもよく特集されますね。人類に生物学的な起源があることに気づいたダーウィン以来、私たちの人類学的な意味での先祖がどういうものだったのかということは、絶えず社会的な関心の的ですね。

ホモ・サピエンスに至る過程は、昔は猿人、原人、旧人、新人という区分があったんですが、今はもう分類としては正しくないということになっていて、学界では基本的には使いません。でも、日本では便利なので言葉としては使っています。

これは、どれかの種を１つ指し示すという意味ではなく、例えば一番古い時期の人類を、「猿人」と呼んでおこうというように、ある段階を表す言葉として使っていて、その限りでは今でも有効だとは思います。

現在から振り返れば何らかの段階を設定することができるけれど、あくまでサピエンスに至るにはこういうステップがあった、みたいな感じですよね。

分かりやすいけれど、ともすれば、進化の頂点がわれわれ、今の人類であると受け取られかねない部分もあると感じます。

今、テレビなどでも、盛んに進化の過程は一本道じゃなかったと言う。それはその通りですが、「たくさん人類がいて、生き残ったのはホモ・サピエンスだけだ」という言い方はちょっとミスリーディングです。

いろいろな種類がいたかどうかについても、多くの議論があります。仮にいたとしても、ご先祖さまに当たる一部はその途中段階なわけです。だから生き残ったのはホモ・サピエンスと言っても、その前がなければ存在できないので、「20種もいて、生き残ったのは１種だけです」と言われると、「ちょっと待って」と言いたくなりますね。

猿人、原人、旧人、新人とだんだん進化してきたわけでもなく、併存していたときもありますし。

そうです。逆に、やたらといっぱい併存していたというのを強調されるのもどうかと。

その時々によって、強調したいことが違うみたいですね。

種を増やすほうが、話題としては面白いので、今は「増やす派」の人が、世間の関心をうまく集められているのではと思います（笑）。

私は機械工学が専門ですが、機械として人間を見たときに、人間ってすごいな、人間のような機械を実現することは難しいけれど、それに挑戦していくことが機械工学として大事だなと思い、人類学の分野をやっています。

突き詰めて考えると、人間はやはり進化の産物ということになるわけです。そういう視点から人間の動きの仕組みを考えていくことは、すごく重要で面白いと思うのです。

機械工学的に面白いのは二足歩行ですか。

そうですね。人類の定義は二足歩行を始めたこと、と認識されていますし。

そんなこともないでしょう。そういう定義もできるけれど、先に「人の起源はチンパンジーと分岐したところです」と言っておいて、「二足歩行は結果的に始まっていました」というのが今の解釈です。二足歩行をしていない人類祖先というのも少しだけでもいるはずなんです。

それはそうだと思いますが、じゃあ、どうそれを見分けるのか。

それはまた別の問題です。今は見分けられないですね。

結局、どう認識できるかの問題ですよね。年代が古い骨がアフリカから出てきて、どうやら二足歩行をしていたようだとなったら、「それは人類だろう」となると思います。

確かに系統学の観点からすると河野さんがおっしゃった通りだと思うけど、実際問題としては「二足歩行をしていた」ということを、定義として用いることにはなると思います。

逆に、二足歩行をしていたかどうか分からないけど、「これは人類だ」と言い得る材料は、初期人類だとどういうところなんですか？

犬歯の縮小ですね。人類は犬歯が小さく、かつオス、メスの差がありません。チンパンジーなどは、犬歯が大きくて、雌雄差がありますが、それがだんだん小さくなっていくのが人間なんです。

だけどそれを証明するのは大変なんです。化石を１個見つけて「小さいから」と言っても、「メスでしょう」と言われておしまいになります。

ある程度分布が分からないといけない。

そうです。440万年前のアルディピテクス・ラミダス（ラミダス猿人）の犬歯は20個以上見つかっていますが、１つも大型のものがなく、皆、メス相当の大きさしかありません。そこで、すでに犬歯は小さくなっていたと主張しています。

20個見つかってその20個が片一方しか出てこない確率は限りなくゼロに近いので、そういう結論になる。でも20個見つけないとそういうことは言えない。

人類学も突き詰めれば、やはり頻度の科学なんですね。最終的には頻度がどれだけ違うかを検討して、「ちょっとあり得ない？」と、初めて結論が出てくる。

でも、人類学では１点物の化石も多いので、それが言えないことも多い。そこを段々と発見することでその間を埋めて、何とかサイエンスとして妥当な解釈をしていこうという果てしない努力ですよね。

化石をベースにした学問だと、数が少ないのが当たり前なので、ばらつきを持った生物群の中に違いが本当にあるのか、ないのか、厳密な意味で言うと難しい。だから、証拠を積み上げて、いかにももっともらしいものにする（笑）。

もっともらしいと言っても、「とんでも」は駄目なので。

「可能な限り正しい」ということですね。だからしょっちゅう説が書き換わることになります。今までなかった化石が出てくると考え方が変わってくる。そういう意味ではダイナミックで面白いと思います。

同じ種類でも、当然、個体差がありますし。２つの化石が個体差の範囲なのか、違う種類なのか、その判断基準がない。

昔の高校の教科書では、原人はシナントロプス・ペキネンシス（北京原人）やピテカントロプス・エレクトス（ジャワ原人）とそれぞれ別種の名前がついていました。それがいつからかホモ・エレクトスで統一するという話になっていきましたね。

化石などがそれなりに増えて、比較するとそこまで違わないんじゃないかと、種をまとめるようなムーブメントが起こるときもある。

一方で、ホモ・エレクトスとしてまとめられていたアフリカ産の化石が、あまりにも違うので、これはまた外しましょうという動きもある。だからもう寄ったり、分かれたりを繰り返している。結局、どう分けるかは人が決めることなので。

サピエンス以前の人類といったときに、日本だと一般の人は、まず北京原人あたりを思い浮かべるかもしれませんね。ネアンデルタールはヨーロッパのイメージが強いので。

ネアンデルタールはシベリア南部までは来ていたんですね。中国やインドでも、いわゆる旧人クラスと言われるようなものが出ているけれど、ネアンデルタールとの関係はよく分かっていませんよね。

アジアは原人の化石がよく出ますね。旧人はあるにはありますが、どこに当てはめるべきかを判断するに足りるほどには出ていない。

荻原さんに伺いますが、北京原人やネアンデルタールというのは、現世人類から見てメカニクス的に際立った違いはあるのですか。

どのレベルで違いを見るかということになりますが、マクロに見ると、肉体的に劇的に違っているとは考えていません。

やはり初期人類である猿人、さらにその前、二足歩行が始まったかどうかあたりとの違いは面白いんですが、原人から先は、正直、私はあまり変わらないと思っています。

筋肉の発達度合いみたいなものはどうでしょう？

そんなに違わないのではないでしょうか。個々の違いはあっても、たぶんバリエーションの範疇に収まってしまう。原人だと上投げ、つまりオーバースローもできる。要するに現代人と基本的には近いんです。

猿人は、上投げはできなかったんですか？

そう言われています。少なくともチンパンジーは、上手に上投げをできないですね。

野球のピッチャーみたいにテイクバックができないのですね。

そうです。肩の構造、肩甲骨の形がずいぶん違っています。『猿の惑星』の猿は上からやりを投げますが（笑）、チンパンジーは基本的には下投げでしょう。原人レベルからできるようになると思っています。

原人とサピエンスでは歩行自体もそれほど変わらないのですか。

そうですね。違っているとは考えられない気がします。スピードもそれほど違わないでしょう。最近人間は持久走に適しているという話がありますが、それも大体原人レベルからと言われています。

ＮＨＫの番組「人類誕生」で言っていたダニエル・リーバーマンの説ですね。

そうです。ホモ・エレクトスのあたりから、獲物を長く追って仕留めることができるようになった。だから人間は決してスプリンターではないけれども、他の動物より持久力に適しているという説です。

それこそ下肢の構造が原人になった頃からより現代人的になってきたということでしょう。正直、走るのと歩くのとどちらに適しているかの見分けは難しいところはあります。

人間の足全体に対して、ありとあらゆるところを、「これは全部走行適応だ」みたいな話もありますが、それはどうかと思っています。

毛がなくなって発汗することで熱がこもらなくなったので、長い距離を走り続けても熱中症になりません、という話もありましたね。

あれはどうやって毛がないと分かるのですか。

それはそういう想定です（笑）。根拠はどこにもないですね。

ＣＧで古い人類を毛がないバージョンと毛があるバージョンで再現したら、我々は認知のレベルでヒトか獣かというくらいに感じてしまいますよね。印象が全然違う。

でも原人になると、汗腺が発達して毛がなくなり、それで放熱が上手くできるということが、走ることへの適応だと考えられている要因なのではないですか？

でも、ＮＨＫではホモ・ハビリス（最初期の原人）は毛むくじゃらでしたね。

ハビリスとエレクトスの間に線があるんですかね。ハビリスは、『2001年宇宙の旅』でモノリスに出遭った頃の人たちですね。300万年前の想定ですから。

なぜハビリスは毛があるということになったのでしょう？

それはエレクトスが長く走るという話にしたいから、そこで初めて毛のない状態になったとなるのでしょう。授業で同じ番組を６回ぐらい見たので頭に染み込んでいます（笑）。もっと前に毛がなくなっていたことを否定する理由もなければ、その時点で毛があった可能性もあります。

エレクトスの頃は、もう完全に地上で生活してサバンナに出ていて、という背景にするから毛をなくしたんでしょうね。

It's an analogy that they must have at least sweated to survive.

Can we tell whether they had hair or not from DNA? It seems there are many things we've come to understand through DNA.

Erectus is likely too old for DNA analysis.

Phylogenetic relationships have become very clear. In other words, we can tell which species are more closely related to each other.

We can also see the order in which they branched off.

Basically, with fossils, we talk about morphological similarities, but when we examine DNA, we often find cases where they look similar but are actually different.

For things that can only be identified by fossils, there's not much we can do. If it's as recent as Neanderthals, we can extract DNA. Even though we call them fossils, the bones haven't turned to stone yet. But before that, they are stone, so we probably can't get DNA.

The oldest DNA recovered so far is from Homo heidelbergensis, dating back 400,000 years.

In the past, there was the multiregional evolution hypothesis for Sapiens, but now the Recent African Origin hypothesis is well-established.

Basically, yes.

That's exactly what DNA has revealed.

Homo erectus is also considered to have originated in Africa.

Are Neanderthals perhaps the only ones that might have originated outside of Africa?

Even so, the ancestral species of Neanderthals was Heidelbergensis, and if you trace back further, the image is that they ultimately came out of Africa.

Yes, I think it ultimately comes down to an African origin.

The Recent African Origin hypothesis refers to the origin of Sapiens, but it's striking that other early humans and close relatives of humans all trace back to Africa.

But even if we go back to the great apes, do they also originate in Africa?

It is believed that all apes, if traced back to their roots, originated in Africa.

Is that so? Mr. Ogihara, you are researching Nakalipithecus, a 10-million-year-old fossil ape found by Kyoto University in Kenya, right?

Unfortunately, I'm not directly involved with that one. What was found was a jaw (laughs).

Kyoto University discovered a 15-million-year-old fossil ape called Nacholapithecus, and for that one, they found almost all the bones of the body. However, fossils from the period close to the divergence between humans and apes are mostly teeth and jaws. It's difficult to estimate locomotion patterns from just a jaw.

Regarding teeth, for apes, various specimens have been found sporadically from about 30 million years ago until just before the emergence of humans. However, it's not clear which ones are in the direct ancestral line of humans.

Early apes have simple teeth, and the only real difference is that at some point, the enamel gets a bit thicker and they become more ape-like.

What are the points that make you think, "This is an ape"?

That would be the shape of the teeth. If you look at the teeth, you can tell it's an ape.

So early apes had ape-like teeth, but their bodies were basically monkey-like.

When you go back to Proconsul (an extinct fossil primate) from 25 million years ago, quite a few body bones have been found. We call them "dental apes" because while the teeth are ape-like, you can't tell just by looking at the body.

Basically, they were monkeys that walked on all fours in the trees.

We assume that the evolution of tooth shape and changes in enamel thickness were caused by diet. However, recently there's been a trend of thought suggesting that enamel thickness is more evolutionarily plastic than previously thought, and it might be impossible to judge whether something is human based on that alone.

I've visited many sites where Asian hominids were excavated, but Homo floresiensis is fascinating. There's a reconstruction model at the National Museum of Nature and Science, and everyone was shocked when the paper was published in Nature in 2004. They were only 110 centimeters tall.

That was a real surprise.

When you actually interact with a 110-centimeter child, you realize how shockingly small that is. Even some kindergarteners are over 110 centimeters.

One wonders if an early human could really become that small.

And the brain was also very small.

On par with a chimpanzee?

Yes. And yet they were as recent as tens of thousands of years ago—it's baffling. It meant the theory that human brains have consistently grown larger isn't always the case. Other early humans were all around 160 centimeters, right?

At the museum, there's a 1.5-million-year-old African Homo erectus boy called Turkana Boy; even as a boy, he's about 165 centimeters. Even for Java Man, it's said there were individuals over 170 centimeters based on femur estimates.

Early humans were large. Why did Homo floresiensis become so small? They must have started large and then shrunk.

It's thought that they crossed to the island and became smaller there.

Does that mean they adapted?

Yes. In an island environment, there are no predators and food is limited, so there's no need or opportunity to grow large, and they gradually shrink. There's a phenomenon called insular dwarfism, and it's said this might be a dwarfed version of a human species.

But some people aren't convinced. They say, "Humans don't get smaller. Brains don't get smaller" (laughs).

There are various people, like those who insist it's just a diseased Homo sapiens.

The most straightforward interpretation is that Java Man was nearby, and some went there and became smaller.

But even though they were that small, their stone tool techniques weren't primitive, were they?

Exactly.

Maybe brain size isn't actually that necessary? (laughs)

Maybe the cells themselves became smaller? With the number of cells remaining the same.

But doesn't cell size not vary that much between species?

That's true (laughs). Whales, elephants, and mice all have the same size cells.

It's purely mysterious.

There was a period of 40 or 50 years where only stone tools were found, and then the bones were discovered later. At first, even when stone tools were reported, many people didn't take it seriously.

Yousuke Kaifu, who is now on the research team looking at the bones, also said, "Even when fossils were reported, I didn't take it seriously at first" (laughs).

In Southeast Asia, there are places on Sulawesi and Luzon where only stone tools have been found, and the bones of the makers haven't appeared yet. Chronologically, they should be close to the era of Homo floresiensis, so it's interesting to see what kind of bones will turn up.

The first major turning point was when early humans eventually became bipedal and their canines became smaller. The next major turning point was likely when the brains of the genus Homo began to grow larger, somewhere between 3 and 2 million years ago.

We call those with large brains the genus Homo. It's said the brain grew larger because they started eating meat. But this is a circular argument. They could eat meat because they used tools, which made the brain larger, but why could they make tools? Because they had large brains—it's a cycle.

However, as with Homo floresiensis, there are cases where they used tools despite having small brains.

But they are still genus Homo. The definition breaks down, but there's nothing else to call them.

I suppose so.

You can't really split them at the genus level like that. Cladistically, if you split that one off, you'd have to dismantle all the others.

In any case, those two phases are where the major changes occurred. However, it's not yet clear whether it progressed at a steady pace or changed rapidly at some point.

Regarding the brain, we know to some extent the pace at which it grew; NHK said it grew significantly once they became Erectus.

NHK works very hard to create proper programs. That's for sure, but it doesn't mean everything is correct. I hope you can view it as one way of understanding the theories currently out there.

The general public tends to think that a standard interpretation has been settled, but that's certainly not the case.

We often speak of a "replacement" of Neanderthals by Homo sapiens, but how should we interpret this? While the term "replacement" was frequently used in the past, recent DNA research is providing continuous evidence supporting interbreeding, so I feel a sense of unease with the word replacement.

Also, regarding archaeological evidence, it used to be said that relatively advanced abstract expressions and artifacts related to art did not appear until Homo sapiens, but recently there have been more reports of evidence showing that Neanderthals also possessed such abilities.

In the past, many people wanted to tell a dramatic story where Neanderthals lost in a war.

That's true. We don't hear that much lately, though. To be honest, we don't know what the most plausible hypothesis is.

The size of Neanderthals and Homo sapiens is almost the same. Anatomically, they are almost identical, but the shape of the head is different.

Since the body of a Neanderthal is slightly larger, the general consensus is that the absolute size of the brain was also slightly larger in Neanderthals. However, I believe that Sapiens may have a relatively larger cerebellum. As for where Neanderthals are larger, it is the occipital lobe around the back of the head, which is the area primarily responsible for vision. However, it is also true that these are subtle differences.

Thinking simply, is it possible that Neanderthals lived a more vision-dominant lifestyle?

Papers have been published suggesting that because Neanderthals lived in relatively high latitudes, they might have lived in dark environments. But we don't really know.

It is probably true that the cerebellum of Homo sapiens is large, but it is debatable whether there is a meaningful difference there. So, if asked whether the difference in brain shape was a factor in the replacement drama, I can't say anything definitive.

I imagine that a large occipital lobe or a large frontal lobe would be directly reflected in the shape of the skull, but does it show up even for something as small as the cerebellum?

It does show up. The shape around the lower part of the back of the head is smaller. We performed a task where we transformed a human brain to fit into a Neanderthal head.

When we calculated the mathematical transformation to fit the human brain in, we noticed that the fit around the cerebellum was poor. At first, we were working under the preconception that differences would appear in the frontal lobe or areas related to higher-order functions, but in fact, the area with the most different shape was the lower part of the back of the head.

So the frontal lobe didn't change that much.

As a result, yes. Looking at the head as a whole, the shape around the forehead is quite different between Neanderthals and modern humans. Because of that, I thought the area around the frontal lobe might be different, but when we compared the shape of the cranial cavity, there wasn't much difference.

The largest Neanderthal head was found by a team from the University of Tokyo, wasn't it?

That's right. It was discovered in a cave in Israel. It's 1740 cc. It's extremely large, but that individual was also tall. Generally, Neanderthals are said to be larger and more thick-boned.

Since the cerebellum is a place deeply related to motor skills, you might think that a small cerebellum means low motor skills, but we don't think so. The cerebellum has many connections with the cerebrum and is also related to higher-order functions, so we believe it was linked to differences in cognitive function rather than motor nerves.

It is said that Neanderthal teeth have thinner enamel for some unknown reason. But it might have just happened to shift in that direction.

There is a story that Neanderthals had heavy wear on their front teeth. It's also said they might have used their front teeth to tan hides.

Modern humans also vary a lot depending on the era and region. As times become more recent, for example, if a division of labor occurs within the same group, it can happen that one person's teeth are worn down while another's are not.

Which DNA was most different between Neanderthals and Sapiens?

I believe there was a story that genes related to language and brain development were slightly different.

Their skin color was light, wasn't it?

Yes. It is said that interbreeding occurred several times.

How can we estimate how many times?

To be honest, I don't know very well.

I want you to tell me (laughs). But it seems there is evidence that interbreeding occurred.

Also, the Denisovans were contemporaries of Neanderthals, and only a pinky bone and two teeth have been found, but when DNA was taken from them, it turned out there was another species that was different from both Neanderthals and Sapiens.

I hear that people researching DNA are saying various things, such as that people around Melanesia have inherited genetic material from Denisovans, or that the high-altitude adaptation genes of Tibetans come from Denisovans.

It's becoming a surprisingly big story.

Yes. And the locations are spreading all over the world.

Well, it's a fact that Neanderthals and Homo sapiens coexisted during the same period. Whether it was all at once or gradually, we don't know, but Neanderthals disappeared.

They overlapped for tens of thousands of years, which is long if you call it long. Homo sapiens was born in Africa about 200,000 years ago, but it's said they didn't fully leave Africa until 60,000 or 70,000 years ago. If they coexisted after that, it would be during the interval since Neanderthals were only around until 30,000 or 40,000 years ago.

Looking at the results, Homo sapiens survived, but many people see the title of the book I wrote recently, "Why Are We the Only Ones?", and think the answer is written there (laughs).

I intended it as a way to set the question. For example, there are many possible explanations for why I am in this meeting room right now. It's probably impossible to narrow it down to one. There must be an element of chance as well.

I suppose we can say that humanity, which had been diverse until then, became a single species after the Neanderthals disappeared. We can also say as a result that migration was active. Because they advanced into places where previous humans had not gone.

Without geographical isolation, they become a single species. If one reason they became a single species is that the population exploded and spread throughout the world, then why did the population explode so much?

Perhaps it wasn't a romantic story like "they could go anywhere," but rather that the population growth became so significant that they had no choice but to leave.

In the first place, when it comes to why the population growth occurred, I wonder if it stems from cooperation or social intelligence. I don't think things like agriculture could be done unless getting along with others was at the root of it.

Agriculture doesn't start until about 10,000 years ago, but they had already reached Patagonia during the hunter-gatherer era.

I think there was some kind of cooperative behavior, and that contributed to increasing the survival rate.

So, if the story becomes that cooperative play was necessary to hunt mammoths, would that be too simplistic? (laughs). Like that being related to the cerebellum. One can imagine many things.

Since hunting by endurance running from the time of Erectus probably linked to doing things in groups, we can also consider that the increase in brain size from that time is related.

In that case, Homo floresiensis is more of a mystery than ever.

It is said that Homo sapiens arrived in the Japanese archipelago about 38,000 years ago. Unfortunately, no evidence of earlier people, such as Peking Man on the continent, has been found to this day.

Sites where stone tools and such are found appear nationwide from about 38,000 years ago. However, almost no bones are found. Due to the nature of the soil in the Japanese archipelago, bones are difficult to preserve.

The exception is Okinawa. Since Okinawa has limestone soil, bones are easily preserved. Paleolithic bones have been found in about 10 locations in Okinawa, and only one location on the main island of Honshu.

Where on Honshu?

The Hamakita site in Shizuoka. It used to be thought there were more, but upon reviewing the dates, they turned out to be more recent, or were judged not to be human bones.

In Japan, archaeology and anthropology are separate disciplines, but in fact, when learning about humans of ancient times, people in other countries often work together. For example, if you want to know about Neanderthals, you need both an archaeological approach and an approach from bone morphology.

In Japan, they are split sharply from the moment you enter university, and I think it's unhealthy that there isn't much interaction and they are thought of as separate disciplines. Do you feel a gap in that regard, Mr. Kono?

It's true that there is a distance from archaeology. However, more than that, the anthropology industry has shrunk, and jobs and positions are decreasing. In the past, quite a few anthropology professors belonged to the Department of Anatomy in the School of Medicine, but now there are fewer. Originally, there weren't that many universities capable of training students anyway.

On the other hand, when bones appear or when there's a special feature on an NHK program, everyone is happy to watch (laughs).

Your current affiliation, Mr. Kono, is the Faculty of Letters, isn't it? Anthropology within a Faculty of Letters is relatively rare.

But anthropology is a field of science, and while it's not immediately useful, it deals with the essence of what a human being is, so it connects somewhere to what people in the Faculty of Letters do.

At first, I wondered why I was talking about this to students in the Faculty of Letters, but when I had them write their impressions, they wrote things like, "I thought it was important for humans to use tools and have culture, but I was surprised to find that the body structure is the same as other primates," so I thought, "I'm glad."

I think it's good that people who will be thinking about humans in the future also look at humans from a biological perspective. Occasionally, there are even students who want to study bones.

Are there any students at Professor Ogihara's place who are interested in anthropology?

Basically, there aren't many, but the discipline that research on the evolution of human bipedalism is based on overlaps considerably with things like robotics.

Originally, I think there is a foundation where if robotics were also done with people who have a biological perspective, something a little different would be created.

I think it's a field that the general public is also interested in, so I hope the number of students interested in anthropology increases.