Nature Podcast | 海绵化石：最古老的动物化石？

又到了每周一次的 Nature Podcast 时间了！欢迎收听本周由Benjamin Thompson和Noah Baker带来的一周科学故事，本期播客片段里, 几位研究人员就Elizabeth Turner最新发现的8.9亿年前的海绵化石展开了讨论，如果如这篇论文所说，这个海绵化石将比之前发现的无争议的最早动物化石还要早3亿多年。欢迎前往iTunes或你喜欢的其他播客平台下载完整版，随时随地收听一周科研新鲜事。

音频文本：

Host: Noah Baker

First up on the show, you might remember that last week we had a storyabout sponges. Well, it seems we’re not done. We have another sponge storycoming up now, albeit from much further in the past. Here’s reporter Nick Petrić Howe with more.

Interviewer: Nick Petrić Howe

When did the first animals evolve? It’s a more difficult question than you might think.

 

Interviewee: Rachel Wood

Well, to be honest, we don’t understand when animals arose at all.

 

Interviewer: Nick Petrić Howe

This is Rachel Wood, a geologist with an interest in the evolution of life.

 

Interviewee: Rachel Wood

And this is for two reasons. First of all, the fossil record of animalsis incredibly difficult to decipher. We have fossils which we,being palaeontologists and geologists, can put our hands on our hearts and say,'This is definitely a fossil animal.’ And certainly, many, many fossils in the Cambrian are undoubtedly animals. So, the Cambrian started at approximately 540 million years ago, so anything younger than that we can be pretty secure is an animal. But going back into older rocks, it gets more and more uncertain.

 

Interviewer: Nick Petrić Howe

Scientists are pretty confident that some fossils from around 550 million years ago are animals. But much earlier than that and things get much more murky. And hang on, didn’t Rachel say there were two major problems?

 

Interviewee: Rachel Wood

The other set of the problems – as if that wasn’t enough– is that the other way we have of trying to work out the origin of major changes in the evolution of life is to use what’s called molecular phylogeny. So, this is really just a family tree based on relatedness of DNA, and when you do this forelucidating the origin of animals, most molecular phylogenies suggest that the origin of animals was anywhere between approximately 650 and 850 million years.

 

Interviewer: Nick Petrić Howe

There are also many theories for what the earliest animals may have looked like. One points to sponges.

 

Interviewee: Rachel Wood

One of the reasons that sponges, in particular, are such a focus of interest is that they’re often accepted to be the most basal ofanimals.

 

Interviewer: Nick Petrić Howe

Sponges – and, yes, they are animals, think about that next time you’rein the shower – don’t have complicated nervous, circulatory or digestive systems. They survive by filter feeding from water. They also have far fewer cell types than other animals, so they may not be a bad place for an evolutionary start. And this week in Nature, there’s a paper thatclaimsto have found a sponge from 890 million years ago. Here’s ElizabethTurner, thepaper author.

 

Interviewee: Elizabeth Turner

It consists of little tubules, okay, so they are 31 thousandths of amillimetre across, approximately, and they’re filled with little calcic crystals that are clear and translucent, and these little tubules are embedded in agroundmass of much more finely crystalline calcic crystals, so they look darker. So, what you see is these little wormy tubes inside adarker groundmass, and what’s important about the little tubes is that they anastomose in a very distinctive way. So, they form a three-dimensional meshwork of divergent branching and re-joining three-dimensionally, which is quite a complex microstructure that can’t be explained as being any of the other possible things that could have been around at the time like fungi oralgae or bacteria.

 

Interviewer: Nick Petrić Howe

These little wormy tubes branch or anastomose in such away that Elizabeth believes it points towards an animal origin – a sponge. Elizabeth believes that these fossils look very similar to some more recent fossils also thought to be sponges.

 

Interviewee: Robert Riding

I mean, I’ve got a picture on my screen now and I’ve gotthe two fossils side by side – the ones that we described 500 million years ago and the one that Elizabeth has found – and I honestly don’t see a difference in them. They look to be identical to me.

 

Interviewer: Nick Petrić Howe

This is Robert Riding, another ancient sponge-hunter describing a fossilsponge he recently published a paper about.

 

Interviewee: Robert Riding

We were looking at rocks that are much younger, about 500 million years old, and they’d always been thought to be stromatolites, which are calcifiedmicrobial mats like you see at present day in places like SharkBay inAustralia. But when you look at them closely, you see this delicate network of tubes which we are convinced are sponge fabrics, and they are interlayered with the stromatolite fabrics or the bacterial fabric. And the sponge fabrics in those are very similar. I would say they’re identical to those that Elizabethhas found.

 

Interviewer: Nick Petrić Howe

Robert and Elizabeth suggest that their fossils represent sponges that would have had a close relationship with bacteria. Infact, in the harsh 890-million-year-old world, they think a relationship like this would have been necessary.

 

Interviewee: Elizabeth Turner

So, they lives in a reef, okay. So, these reefs were built by photosynthesising organisms. And this is important because, at the time, 890million years or so ago, Earth didn’t have a whole lot of oxygen in its atmosphere or its ocean, and animals obligatorily require a certain amount of dissolved oxygen in the water or in the atmosphere. And in fact, where I findthem living is in the little pockets and crevices, little tiny caves just underneath the reef surface, little caves that are centimetres in diameter.

 

Interviewer: Nick Petrić Howe

Also, if these fossils are indeed sponges that lives 890million years ago, that would mean that they survived some of the harshest parts of Earth’s history – a very cold period known as the Cryogenian, where it is possible that nearly all of Earth’s surface was frozen, referred to by some as'Snowball Earth’.

 

Interviewee: Robert Riding

So, that gives you, in a way, a handle on the glaciations. It means that there must have been life, even if we haven’t found it yet. There must have been animal life surviving the Snowball Earth glaciations.

 

Interviewer: Nick Petrić Howe

But as you might remember from the beginning, thefurther back in time you go, the more uncertainty. This fossil would predate any uncontentious animal fossils by several hundred million years. So, how likely is this to really be an animal fossil? Here’s Rachel again.

 

Interviewee: Rachel Wood

So, first of all, I’d like to say it’s great that this idea has been proposed. It’s going to cause a huge amount of discussion and debate, no doubt heated discussion and debate and controversy. The author is proposing that this is a sponge fossil based on its similarity to very, very similar fossils that are found in much younger rocks, but the problem is are these younger fossils themselves sponges? When you read the literature on this, it is actually slightly tentative. The proposal is that they are interpreted as sponges.

 

Interviewer: Nick Petrić Howe

Rachel, who wasn’t associated with this latest research, believes that there are other possible interpretations for this fossil.

 

Interviewee: Rachel Wood

I think a bacterial origin or some sort of general microbial origin, an origin of structures formed by biofilms, which are these sort of consortia orcommunities of different microbes that form slimy surfaces,they can create these interesting and seemingly complex microstructures. So, I think all this suite of microbial precipitate needs to be explored.

 

Interviewer: Nick Petrić Howe

On the other hand, the paper author Elizabeth is quite certain about her sponge interpretation.

 

Interviewee: Elizabeth Turner

Me, I’m quite confident, and that’s because I’m a carbonate petrographer. One of the things I understand really well in this worldis limestones and how to understand the evidence of life that’s preserved in them.That’s what I do. And so, this particular type of microstructure that I see and that I’ve documented is very, very well documented now in younger rocks and not at all controversial in the younger rocks. The material I’ve identified identical to the younger stuff, so I mean it’s almost a no-brainer.

 

Interviewer: Nick Petrić Howe

Robert too thinks that sponge is the most likely conclusion.

 

Interviewee: Robert Riding

Let’s go for 99.9% because I can’t tell you what the difference is. The only difference I know is in age. Otherwise they look identical. So, I’m really very strongly convinced.

 

Interviewer: Nick Petrić Howe

Everyone I spoke to for this story did agree, however,that there will be a debate regarding this finding. But that could stimulate more research to try and track down animals’ early ancestors. In fact, they seemed mostly enthusiastic about this. After all, scientific debate can be agreat catalyst for discovery.

Interviewee: Elizabeth Turner

By proposing this identity for a material this old, I’m already putting myself into an area of rather pronounced controversy. There are people who really won’t like this and that’s fine. I’m just throwing it out there, folks. Discuss, right?

 

Host: Noah Baker

That was Elizabeth Turner from Laurentian University inCanada. You also heard from Robert Riding from the University of Tennessee,Knoxville in the United States and Rachel Wood from the University of Edinburghin the UK. You can read more about this discussion in the paper published this week in Nature. We’ll put a link in the show notes. And Nick has been very busy – there’s also a video all about early animals that has gone on our YouTube channel. Check the show notes for that too.

《自然》论文：

Possible poriferan body fossils in early Neoproterozoic microbial reefs