In a year when a single virus has killed millions, it's hard to think of viruses as anything other than bad.

今年，一個病毒就奪走了數百萬人的生命，人們很難不把病毒當成壞東西。

Our understanding of them is largely based around the death and destruction they cause.

我們對它們的瞭解主要基於它們造成的死亡和破壞。

HIV has caused the deaths of over 33 million people since the start of that epidemic in the 80s.

自上世紀 80 年代艾滋病開始流行以來，已造成 3300 多萬人死亡。

The Ebola virus kills up to 90% of people who get infected with it in a gruesome and awful way.

埃博拉病毒會以可怕的方式殺死 90% 的感染者。

Polio still paralyzes people.

小兒麻痺症仍然使人癱瘓。

Zika, dengue, yellow fever still ravage the tropics.

寨卡、登革熱、黃熱病仍在熱帶地區肆虐。

So from where we're standing, it very much seems that viruses are not our friend.

是以，從我們的角度來看，病毒似乎並不是我們的朋友。

There are an estimated quadrillion quadrillion individual viruses, 10 to the 31st power, that exist on this earth with than all the stars in the universe.

據估計，地球上存在的四萬萬億個病毒（10 的 31 次方）比宇宙中所有的恆星還多。

But they aren't all bad.

但它們並不都是壞的。

Only a small few have the ability to cause us harm, and the vast majority don't affect us at all.

只有極少數有能力對我們造成傷害，絕大多數根本不會影響我們。

But over time, some have become beneficial to us, in fact, a part of us.

但隨著時間的推移，有些已成為對我們有益的東西，實際上是我們的一部分。

Viruses have been infecting us and our ancestors for as long as we've been around.

從有人類以來，病毒就一直在感染我們和我們的祖先。

And around a hundred million years ago, our mammalian ancestor was from that point on.

大約一億年前，我們的哺乳動物祖先就是從那個時候開始的。

The traces of this infection, and many others, exist as viral fossils within our genome.

這種感染以及其他許多感染的痕跡，就像病毒化石一樣存在於我們的基因組中。

Around 8% of our DNA is not human at all, but viral.

我們的 DNA 中大約有 8% 根本不是人類的，而是病毒的。

And in recent years, scientists have started to see that these viral gene sequences are not simply leftover genetic baggage, strange and purposeless, but are sequences that code for essential proteins in human development.

近年來，科學家們開始發現，這些病毒基因序列並不只是遺留下來的基因包袱，既奇怪又毫無用處，而是人類發育過程中重要蛋白質的編碼序列。

How can it getting into our genome?

它如何進入我們的基因組？

How did these viral segments of DNA become a part of us?

這些 DNA 病毒片段是如何成為我們的一部分的？

And what do they do for us that's so important?

他們為我們做了什麼如此重要的事情？

No one knows for sure where viruses came from, whether they existed before any cells did, assembling themselves from molecules in the environment, or whether they are derived from cellular life, leaking out of or being reduced from a viruses are perfect parasites.

沒有人知道病毒從何而來，它們是在細胞出現之前就已經存在，利用環境中的分子組裝而成，還是源於細胞生命，從病毒中洩露出來或被還原成病毒，病毒是完美的寄生蟲。

They are tiny particles of genetic material enclosed in a protein coating, and their existence depends on invading cells to hijack the cellular machinery to replicate themselves.

它們是包裹在蛋白質塗層中的微小遺傳物質顆粒，其存在依賴於入侵細胞劫持細胞機器來複制自身。

To invade a cell, a virus recognizes and binds to the cell via a receptor molecule on the cell surface, and then breaks in.

為了入侵細胞，病毒會通過細胞表面的受體分子識別並與細胞結合，然後侵入細胞。

Once inside, the viral genes are expressed, viral proteins are created, and new virus particles are assembled and are ready to be released.

一旦進入體內，病毒基因就會被表達，病毒蛋白就會被生成，新的病毒顆粒就會被組裝起來，隨時準備釋放出來。

In some cases, the exiting viruses leave the host cell intact so it can continue cranking out more virus particles.

在某些情況下，退出的病毒會完整地留在宿主細胞中，以便繼續製造更多的病毒顆粒。

But sometimes, the viruses cause the host cell to burst, killing it.

但有時，病毒會導致宿主細胞破裂，使其死亡。

The viruses then float within the extracellular space, poised to infect more cells.

然後，病毒漂浮在細胞外空間，準備感染更多的細胞。

This is likely the story of viral infection that you've heard.

這很可能是你聽說過的病毒感染故事。

It's the primary mechanism of spread for most viruses.

這是大多數病毒的主要傳播機制。

But some viruses, like HIV, measles, and the herpes simplex virus, can also spread directly between two cells that are in contact with each other, in a process called cell-to-cell transmission.

但有些病毒，如艾滋病病毒、麻疹病毒和單純皰疹病毒，也可以在兩個相互接觸的細胞之間直接傳播，這一過程被稱為細胞間傳播。

This is particularly insidious, as it enables viruses to evade the body's immune response.

這一點尤其隱蔽，因為它能讓病毒躲避人體的免疫反應。

Viruses that can spread in this way have a few mechanisms to achieve it.

能以這種方式傳播的病毒有幾種實現機制。

Perhaps most notably, genes that code for proteins, that force host cells to fuse together, allowing the viruses to jump from cell to cell.

也許最值得注意的是編碼蛋白質的基因，這些蛋白質能迫使宿主細胞融合在一起，使病毒在細胞間跳躍。

And these genes, that seem absolutely harmful to humankind by allowing some viruses to wreak havoc on our bodies, have also enabled us to exist at all.

這些看似對人類絕對有害的基因讓一些病毒在我們的身體上肆虐，但也讓我們得以存在。

When these particular genes got inserted into our ancestors' genomes a hundred million years ago, the course of human evolution was changed forever.

一億年前，當這些特殊基因被植入我們祖先的基因組時，人類進化的進程就永遠改變了。

The human genome contains around 100,000 fragments of viral DNA, and the story of how it got there begins with retroviruses.

人類基因組包含約 10 萬個病毒 DNA 片段，而人類基因組是如何形成的，這還要從逆轉錄病毒說起。

Retroviruses are viruses that have an RNA genome and work backwards from how other viruses work.

逆轉錄病毒是一種具有 RNA 基因組的病毒，與其他病毒的工作方式相反。

Instead of using DNA to make RNA, which is then used to make transcriptase, this DNA then makes its way to the nucleus and inserts itself into the host cell's DNA, thus changing the genome of the host cell.

這種 DNA 並不是用 DNA 來製造 RNA，然後再用 RNA 來製造轉錄酶，而是進入細胞核，插入宿主細胞的 DNA，從而改變宿主細胞的基因組。

HIV, for example, is a retrovirus that incorporates itself into the genome of a cell, where it lives as a template for creating more HIV viruses.

例如，艾滋病病毒是一種逆轉錄病毒，它將自身融入細胞基因組，作為模板存活在細胞中，以創造更多的艾滋病病毒。

The HIV virus infects immune cells, but some viruses infect the cells that will be passed through the generations.

艾滋病毒會感染免疫細胞，但有些病毒會感染世代相傳的細胞。

And this is where our history with viral genes begins.

我們與病毒基因的歷史就從這裡開始。

If retroviruses infect reproductive cells, the cells that produce eggs or sperm, the virus inserts its DNA into the heritable genome of the host.

如果逆轉錄病毒感染生殖細胞，即產生卵子或精子的細胞，病毒就會將其 DNA 植入宿主的遺傳基因組中。

Once a retrovirus has embedded itself in this way, it's known as an endogenous retrovirus.

一旦逆轉錄病毒以這種方式嵌入體內，它就被稱為內源性逆轉錄病毒。

At first, endogenous retroviruses forced cells to make more retroviruses that can infect other cells.

起初，內源性逆轉錄病毒迫使細胞製造更多可以感染其他細胞的逆轉錄病毒。

But over the generations, the viral DNA mutates, and endogenous retroviruses eventually lose the ability to infect new cells.

但經過幾代人的努力，病毒 DNA 發生了變異，內源性逆轉錄病毒最終失去了感染新細胞的能力。

Much of the viral DNA within our genomes is therefore thought to be dormant, evidence of our ancestors' past infections, but nothing that affects us now.

是以，我們基因組中的大部分病毒 DNA 被認為處於休眠狀態，是我們祖先過去感染過病毒的證據，但現在不會對我們造成任何影響。

But for some endogenous retroviruses, the story is more complicated.

但對於某些內源性逆轉錄病毒來說，情況則更為複雜。

Scientists have realized that even after being disabled and no longer being able to fully replicate itself, some retrovirus sequences can still make some of their proteins.

科學家們已經意識到，即使被禁用，不能再完全複製自身，一些逆轉錄病毒序列仍然可以製造它們的一些蛋白質。

And over time, two of these viral proteins became essential in human development, evolution co-opting viral genes for human needs.

隨著時間的推移，其中兩種病毒蛋白成為人類發育過程中必不可少的物質，進化將病毒基因與人類需求相結合。

In human pregnancy, there are many biological marvels, and the placenta is at the front of that list.

在人類懷孕過程中，有許多生物奇蹟，胎盤就是其中的佼佼者。

It's an organ that develops in the uterus during pregnancy and provides oxygen and nutrients to the growing baby and removes waste products from the baby's blood.

它是懷孕期間在子宮內發育的器官，為正在成長的胎兒提供氧氣和營養，並清除胎兒血液中的廢物。

The placenta attaches to the wall of the uterus and is where the baby's umbilical cord arises from, and it may have never developed without the help of viral proteins since SIDIN-1 and since SIDIN-2.

胎盤附著在子宮壁上，是嬰兒臍帶的來源，自 SIDIN-1 和 SIDIN-2 以來，如果沒有病毒蛋白的幫助，胎盤可能永遠不會發育。

Around five days after fertilization, the two distinct cell types, the inner cell mass, which will develop into the fetus and eventually become the baby, and trophoblasts, which will develop into the placenta and external membranes.

受精後五天左右，兩種不同類型的細胞--內細胞團（將發育成胎兒並最終成為嬰兒）和滋養層細胞（將發育成胎盤和胎膜）--開始發育。

Around the seventh day, the blastocyst starts to implant onto the uterine wall, where it will remain attached until birth.

大約第七天，胚泡開始著床到子宮壁上，它將一直附著在子宮壁上直到出生。

At this point, the syncytiotrophoblast starts to form around the developing placenta.

此時，合胞體開始在發育中的胎盤周圍形成。

It's a structure that determines which substances cross the placenta, such as nutrients and oxygen, and which substances do not, such as certain maternal hormones and toxins.

它的結構決定了哪些物質（如營養物質和氧氣）能穿過胎盤，哪些物質（如某些母體激素和毒素）不能穿過胎盤。

It's both life-giving and protective, and is the structure that results from the once viral syncytin proteins.

它既是生命之源，也是保護之源，是由曾經的病毒性 syncytin 蛋白形成的結構。

In viruses, the syncytin proteins allow them to fuse host cells together, so they can spread from one cell to another in the cell-to-cell viral transmission I syncytin proteins allow placental cells to fuse together to form this vital layer next to the uterus.

在病毒中，syncytin 蛋白使宿主細胞融合在一起，這樣它們就能在細胞間的病毒傳播中從一個細胞傳播到另一個細胞。

Without syncytin, the placenta does not form correctly, and if it had never been introduced into the human gene pool, the placenta would not have evolved as it is today.

沒有 syncytin，胎盤就無法正常形成，如果人類基因庫中沒有引入 syncytin，胎盤就不會進化成今天的樣子。

Without it, internal pregnancy would have developed very differently, or perhaps not at all.

如果沒有它，體內妊娠的發展就會大相徑庭，或者根本不會發展。

Viral sequences that live in our genome act as raw material for new adaptations.

生活在我們基因組中的病毒序列是新適應的原材料。

Lines of code that should be useless to us, turned into something valuable in what feels like a miraculous turn of events.

本該對我們毫無用處的代碼行，卻在奇蹟般的轉折中變成了有價值的東西。

It reminds us that evolution has no plan.

它提醒我們，進化是沒有計劃的。

There is no single track on which it operates.

它的運行軌跡並不單一。

It's a machine of randomness and variability, and in all likelihood, our history of past retroviral infections, and even current ones, will continue to shape our Behind the scenes at Real Science and Real Engineering, we are constantly trying to improve our creativity and workflow to bring you all better and better videos.

在《真實科學與工程》的幕後，我們一直在努力改進我們的創造力和工作流程，為大家帶來越來越好的視頻。

I am personally bad at multitasking and juggling several different projects at once.

我個人不擅長同時處理多個不同的項目。

To help deal with this challenge, I turned to our friend and fellow YouTuber Thomas Frank's productivity class on Skillshare, called Productivity Masterclass, Create a Custom System That Works.

為了幫助應對這一挑戰，我求助於我們的朋友、YouTuber 的同行托馬斯-弗蘭克（Thomas Frank）在 Skillshare 上開設的生產力課程，名為 "生產力大師班，創建一個行之有效的自定義系統"（Productivity Masterclass, Create a Custom System That Works）。

This class helps you create a task management system that you'll actually stick to.

這門課能幫助你創建一個你能真正堅持的任務管理系統。

It taught me how to manage our calendars better, and set up a task management system that I now use religiously.

它教會了我如何更好地管理我們的日曆，並建立了一個任務管理系統，現在我一直在使用這個系統。

And as our team grows, we've also been expanding our animation capabilities, with more and more 3D animations made in Blender and Cinema 4D, with still plenty of 2D and character animations too.

隨著團隊的壯大，我們的動畫製作能力也在不斷增強，使用 Blender 和 Cinema 4D 製作的三維動畫越來越多，二維動畫和角色動畫也有不少。

Skillshare has fantastic courses in multiple animation platforms, like Blender and After Effects.

Skillshare 有多種動畫平臺的精彩課程，如 Blender 和 After Effects。

When I started making videos, I didn't know how to animate, like at all, thinking the software was too intimidating.

剛開始製作視頻時，我根本不知道如何製作動畫，覺得軟件太嚇人了。

But once you dive in, you realize it's not as hard as you thought, especially with the classes Skillshare has.

但是，一旦你投入其中，你就會發現它並不像你想象的那麼難，尤其是 Skillshare 提供的課程。

Right now, I'm working on honing in my skills with this class called Animating with Ease in After Effects, which is teaching me how to use keyframes to their maximum potential.

現在，我正在通過這門名為 "After Effects 中的輕鬆動畫 "的課程磨練自己的技能，這門課教我如何最大限度地利用關鍵幀。

Skillshare is an online learning community, with thousands of people.

Skillshare 是一個擁有數千人的在線學習社區。

It's curated specifically for learning, meaning there are no distracting ads, and it's less than $10 a month with an annual subscription.

它專為學習而設計，沒有任何干擾性廣告，每月訂閱費不到 10 美元。

But the first thousand people to sign up with the link in the description will get a free trial of Skillshare Premium, so there's no risk in trying it out and looking for a class that piques your interest.

但是，使用說明中的鏈接註冊的前一千名用戶將獲得 Skillshare Premium 的免費試用版，是以嘗試一下並尋找自己感興趣的課程沒有任何風險。

As always, thanks for watching, and if you'd like to see more from me, the links to my Twitter, Instagram, and Patreon are below.

一如既往，感謝您的收看，如果您想了解我的更多資訊，請點擊下面的鏈接進入我的 Twitter、Instagram 和 Patreon。