科学60秒：我们的祖先，可能是一只米粒大小的虫子

Tiny Wormlike Creature May Be Our Oldest Known Ancestor

Ikaria wariootia的艺术概念图

图片来源：Sohail Wasif/UCR

大约5.55亿年前，这个两侧对称的生物在海底缓缓爬行，从一端摄入有机质，再从另一端排出废弃物。

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Half a billion years ago, there existed a wormlike creature the size of a grain of rice. And a new study finds that this animal may have been the first to crawl around the seafloor, gobble uporganic matter at one end and poop it out the other end. The creature, dubbed Ikaria wariootia, was probably one of our oldest relatives.

“Ikaria is maybe the oldest bilaterian animal that we find in the fossil record. So this is twice as old or more than things like dinosaurs.” 

University of California, Riverside paleontologist Scott Evans. He says animals like sponges are even more ancient, but they lack the bilateral symmetry that characterizes most animals today.

“So a front and a back and a symmetrical left and right side. And bilaterians also have an opening for food to go in, an opening for waste to go out and a gut connecting them, basically a tube. And really, most animals, everything from insects to mammals to us, those are all bilaterians that are around today.” 

Evans and his colleagues discovered the humble creatures in fossil layers from the Ediacara Hills of South Australia. They used 3-D laser scanners from NASA to make high-resolution images of Ikaria and the surfaces they lived on. The scans confirmed the animals’ bilateral bodymorphology and revealed the shape of the burrows they left from scavenging the seafloor. 

“It probably had a body that was divided into what we refer to as modules, or units or segments. And because of the way it moved through the sediment, we think it had muscles and probably moved similar to how an earthworm moves by sort of extending and contracting its body using those muscle groups.” 

Later animals built off of Ikaria’s basic body morphology—which featured a small front end and a larger rear end.

“It doesn't have a head or a tail, but it’s starting that type of body organization by which things can build a head and a tail.”

What’s more, Ikaria had the ability to sense the environment around it.

“And sense where food was and where oxygen was, which is also a critical evolutionary step in these early animals.”

The study is in the Proceedings of the National Academy of Sciences. [Scott D. Evans et al., Discovery of the oldest bilaterian from the Ediacaran of South Australia]

It might not seem like we share that much with a 555-million-year-old fossil, but looks can be deceiving. There’s probably a little bit of Ikaria in all of us.

—Susanne Bard

 [重难点词汇、短语]

gobble up: 大口吞吃；很快消耗掉；吞并
dub: v. 把……称为
bilaterian: adj. 两侧对称的
morphology: n. 形态学
burrow: n. 地洞；v. 挖掘

 

约5亿年前，一种只有米粒大小的蠕虫状生物曾生活在地球上，一项新的研究发现，这种生物可能是第一种在海底爬行的生物，从身体的一端大口吞食有机质，再从另一端排泄出来。这种被称为Ikaria wariootia的生物或许是我们最古老的亲戚之一。

“Ikaria可能是我们在化石记录中找到的最古老的两侧对称生物。也就是说，它们的历史是恐龙的两倍或更多。”加州大学河滨分校（University of California, Riverside）的古生物学家斯科特·埃文斯（Scott Evans）解释道。他表示，类似海绵的动物甚至更古老，但是它们缺乏现代大部分动物特有的两侧对称特征。

“两侧对称动物有前后端，以及对称的左右侧，还有一个开放的进食口、一个开放的排泄口，和连接两个开口的肠道（通常为管状）。从昆虫到哺乳动物再到人类，这些都是如今的两侧对称动物。”

埃文斯和同事在南澳大利亚爱迪卡拉山的化石层中发现了这种生物。他们利用美国航空航天局（NASA）的3-D激光扫描仪获取了Ikaria及其生存环境的高清图片，扫描图片证实了它们两侧对称的身体特征，揭示了它们在海底爬行搜寻时留下的孔洞形状。

“它或许出现了所谓的身体上的分节。鉴于它在海底沉积物中的活动方式，我们认为它具有肌肉，而且很可能像蚯蚓一样利用肌肉群伸缩身体进行移动。”

后来动物的身形建立在Ikaria的基本身体形态之上，主要特征就是前端小后端大。

“它没有头或尾，但它已经具备了可以构造成头和尾的身体组织。”

除此之外，Ikaria还具备感知周围环境的能力。

“能感知食物和氧气的位置，于早期动物而言是进化重要的一步。”

这项研究发表在《美国国家科学院院刊》（Proceedings of the National Academy of Sciences）上。

或许这块5.55亿年前的化石看上去和我们大不相同，但外表可能具有欺骗性，也许我们每个人都继承了Ikaria的一小部分。